Most of us will admit Disneyland, particularly Main Street, wasn't designed for the daily attendance it receives today. During peak season navigating through Adventureland, New Orleans Square, the Castle Courtyard, Main Street, and the Tomorrowland entrances can be nightmares. While it doesn't bug me too much, I'm positive it's incredibly stressful for those with children and those with autism/claustrophobia. So, here's a mega-thread about "clearing the pipes" (walkways). I have several ideas so feel free to add on.
In terms of guest path selection and utilizing more real estate, I would turn all smoking areas and the Esplanade into vaping areas. Vaping in theory has less collateral damage(perceived and actual) than second-hand smoke so guests should have fewer reservations trekking past a shortcut adjacent to a vaping area. Also, this allows vaping areas to be placed in more/different locations since Disney consciously tried to place smoking areas in low traffic areas. The current picnic area gets turned into the one smoking area for the whole resort and Tom Sawyer Island gets 30-50 wooden picnic tables to replace the current picnic area. My reasoning for taking smoking out of the Esplanade is it makes park hopping more pleasant for families and California Adventure has tons of underutilized capacity.
Critter Country's major problem is underutilized capacity, not crowds. A solution is moving the Splash Mountain Fastpass distribution to the extended queue next to the Haunted Mansion exit and possibly putting Splash Mountain on its own network, similar to California Screamin'. The Hungry Bear and Harbour Galley, like most QSRs in the resort, are underutilized so I'd implement a 40% discount for APs and 3+day ticket holders since they're not in a rush to hit every E-ticket in one or two days. The fresh fruit cart gets moved across to where the Splash Mountain Fastpass machines were. If Harbour Galley gets busy enough, a second story with more seating can be added. The other major concept is to the number of Splash Mountain Fastpasses so that the queue never passes the Briar Patch. If Critter Country can bring in higher counts with discounted food/more Fastpasses, their main pipe will need that extra width from removing the extended queue.
New Orleans Square's pipes aren't clogged too much but sometimes certain factors create a perfect storm. The elephant in the room is Fantasmic! and my half-solution to this is moving enough entertainment, Meet and Greets, and stroller parking to where everyone camps out for Fantasmic! This sends the message,"Don't start camping out until 2-3 hours before." Haunted Mansion doesn't need Fastpass unless it's the NBC overlay while Pirates might eventually need Fastpass due to their extended queue blocking critical pipes as well as the fact that their CMs frequently don't extend the queue fast enough to prevent overflow into Frontierland. The only way to implement Fastpass for Pirates without causing jams near Greeter/Validation and stealing valuable NOS real estate for the machines is Magic Band. This shouldn't happen anytime soon and I'll explain why later. Ideally the French Market's outdoor seating gets a 2nd story but I'm not sure how cost-effective this would be since I'd like the 2nd story to have decent views of the jazz band.
For Frontierland Big Thunder doesn't need more Fastpasses since their extended queue just goes up the empty pipe up Big Thunder trail. The massive space for Big Thunder Barbecue and the Festival Stage get replaced with at least a C-ticket, hopefully an E-ticket since Big Thunder Trail is probably one of the most empty pipes in the park. Depending on an expanded Fantasyland situation, there may be a path connecting the end of the present-day Toontown to Big Thunder Trail/new Frontierland C or E ticket. River Belle Terrace loses some real estate to solve fix the Adventureland/NOS bottleneck.
Adventureland isn't too tricky to fix. The planters in the walkway get demolished for stroller parking, as well as the current Bengal Barbecue and portions of the Indiana Jones Outpost. This makes a much larger pipe and gaps can be placed in the stroller parking for left turns to Indy and Jungle Cruise. The Indy Fastpass machine gets moved to the Outpost so guests don't have to take left turns to get a Fastpass for Indy. An ODV cart/larger Jungle Cruise queue takes the place where the current Indy Fastpass machines are. Bengal Barbecue gets moved to the interior of Aladdin's Oasis, which won't be a problem since they can share a kitchen with Jolly Holiday Bakery. Presently, Indy doesn't really need more Fastpasses since its extended queue doesn't cause jams except possibly when Disneyland is at capacity the week after Christmas. Aladdin and Jasmine can still do reduced Meet and Greets in the Oasis even if the Bengal Barbecue takes up some space.(Space will be limited during lunch and dinner so that's why I say reduced.)
Fantasyland isn't too bad, aside from the courtyard and occasionally the pipe between Teacups and Dumbo. In the short term, Teacups can expand their capacity by increasing staffing to 2 throughout the year, not just the busiest days. Teacups never has an extended queue but additional capacity may be needed if Dumbo and Peter Pan get Fastpass. In the long term, Dumbo and Peter Pan getting Fastpass is a simple way to get rid of queue space and increase pipe size. However, they both have similar problems to Pirates in that they'll cause bottlenecks at Validation unless Magic Bands are implemented. Magic Bands or not, Castle Heraldry gets demolished for the Peter Pan extended queue/stroller parking not in the critical pipe. Matterhorn can easily implement Fastpass without causing any jams but there currently isn't enough demand to justify it.(Hence why some of us are rooting for a Frozen overlay, even if it's only seasonal.) Personally, I think Small World would get higher counts with a shorter flume. The show building has a ton of dead space so even if you cut the flume length by 20-50% would could still fit all 7 continents and most of the Disney characters in. Also, Casey Jr. will eventually need an extra spare train so both trains can be running in the off-season when Dumbo and Peter Pan eventually get their Fastpasses.
I'm not going to touch Tomorrowland thematically but I do have a few ideas in terms of the pipes. If the People Mover truly isn't coming back, it's time to tear out the track from the Observatron to the entrance. Astro Orbiter gets pushed back to the Observatron/Space Mountain extended queue/Magic Eye Theater. The giant rocks also get removed. Thematically, I have no idea how to make an appropriate entrance that doesn't require support beams that block the main Tomorrowland pipe. Star Tours needs more Fastpasses since it's extended queue takes space out of the main pipe. The Jedi Training Academy can ideally be moved into the Magic Eye Theater to where they can do shows every 30 minutes, similar to what Muppets does right now. The Tomorrowland Terrace Stage gets used at night since Nemo, Autopia, and Astro Orbiters' critical pipes contain children who probably have earlier bedtimes than the intended audience for the Tomorrowland Terrace shows. Personally I'd love to replace the Subs with a conservation/species preservation themed clone of The Seas at Epcot.(Yes, we'll keep Nemo for the dark ride portion.) There's enough sunk cost/Subs fanatics to where I'd be fine with just adding Fastpass and using the extended queue for a larger dining area for Tomorrowland Terrace. Tomorrowland Terrace gets replaced with a McDonalds menu and the associated higher volume of burgers and fries output per hour. Disney needs more hotel rooms to pay for a 3rd gate so they could "conveniently" buy the McDonalds on Harbor as part of their acquisition of land for another hotel.(That McDonalds currently isn't much better than park food anyway since they eliminated low-margin and loss leader items from their menu.)
Toontown...I'd personally demolish it for a Fantasyland expansion. In the meantime I'd have no problem expanding the number of Fastpasses for Roger Rabbit and putting it on its own network/the same network as Splash Mountain. I doubt the beancounters could justify this with Toontown's shortened hours but if they extended the Gadget's Go Coaster track ever so slightly they could add a hold position right before the station and run 2 trains.
Main Street has a serious problem during parades. The current solution is just redirecting guests through the backstage area so guests get the magic spoiled by seeing the Jungle Cruise boat storage area right behind the Penny Arcade. My desperate, blasphemous solution to the Main Street bottleneck is an underground passage from beneath the Main Street train station to the hub. This would mean the Partners statue gets demolished, along with that magic of walking down Main Street and the forced perspective of Sleeping Beauty Castle. Still, who are we to force families to inch down a claustrophobic Main Street during parades because we speculate that was Walt's vision? Beancounters will unfortunately love this idea since it means the retail space for Main Street could nearly double. The underground passage could still be incredibly magical with all sorts of lighting effects paying tribute to Disney classics/Main Street U.S.A. The seasonal lighting effects could also be changed more rapidly than most seasonal decorations on Main Street, giving it an extra appeal to APs who visit multiple times a year and expect something new on a regular basis.
As you may have noticed, expanding Fastpass plays a crucial role in the long-term plans. Magic Bands/Fastpass+ are probably the only ways guests will take advantage of the expanded Fastpasses and ride the underutilized attractions, see the underutilized shows, or eat and shop at the underutilized restaurants/stores while waiting for their times. There's one word to describe why I wouldn't bring Fastpass+ to the resort today: smartphones. Currently over 90% of adult guests at Disneyland don't have a smartphone with a battery that will last over 10 hours of constant 3G/4G use. If guests are unable to use their smartphone to get Fastpasses and/or modify their return times they'll swamp the kiosks and clog the pipes. A workaround to this is TDA could issue a heavily locked down smartphone just for Fastpass+ to front line cast members in similar proportions to who they issue pin trading lanyards to. (If you see a cast member with a pin trading lanyard, they're not in a safety critical position and they're probably not super busy.) This would be incredibly expensive but as smartphone costs drop lower everyday it may be viable.