Maybe if they used the Little Mermaid ride seen in the DVD, only spruced up a bit to be elaborate and up to par with the rest of DisneySea. Notice how the other dark rides in the park are elaborate and far from cheap? Sindbad, TSMM, Indiana Jones Adventure's interior scenes, Tower of Terror's interior scenes, Journey to the Center of the Earth, 20,000 Leagues Under the Sea, and...I think that's it. While none of them are really dark rides with slow moving vehicles, they do have dark ride roots with their interior scenes and whatnot.
Space-wise, it might fit (barely) just north east where Flounder's coaster and Scuttle's ride are. But a building there would completely destroy the ambiance since it'll be right at the water's edge. (even if it's themed) and destroy sight lines.
Mermaid Lagoon is indeed amazing, and lots of fun even if you're not a little kid. The "shadow" room in Ariel's Playground is not to be missed. And if you have a kid, then you can easily spend an entire afternoon there going on all five rides, visiting the playground, eating a the restaurant, and shopping. But it gets very busy on a rainy day!
I think the imagineers did a fine job of tying in Midway Mania to the surrounding area, theme wise and architecturally. The brightness and whimsy of the structure bring a good contrast to the existing area. And also create a much needed "weenie" for what was a dead ended avenue. Haters gonna hate..or something (haha) , but it looks top-notch just like everything else at TDL IMO.
---------- Post added 06-12-2012 at 05:35 PM ----------
The website is really fun: when the animation at the top finishes loading and the curtain opens, your cursor will aim the "gun" like in the ride and by clicking on the targets and other objects you shoot a baseball at them.