Just a small thing, but I'm fairly certain that the lack of Viridian Woods in GSC was mainly because of limited data space on the GameBoy cartridges (which is also probably why they had to remove the Pokémon Tower and replace it with a Radio Tower that you could only visit the first floor of, and all those other "barren" Kanto places).
But in a way that kind of ties in with this topic; I mean, why is it that Mauville is the only city that looks so drastically different? It doesn't necessarily look grander, it just looks like it's behind a wall (it's times like these that make me wish we had a really high-quality version of this map to really scrutinize it!). Lilycove and Ever Grande seem to be very similar to their original counterparts, and those two locations are ones I would imagine would be spruced up quite a bit for these remakes. I even Slateport to receive a grander redesign, but with the concept art we've seen it's very safe to say that it hardly changed! Those are a few cities that could really benefit from the hardware upgrade, but they don't seem to be changed from their original iterations.
So if it wasn't "missed opportunities" with the city due to hardware limitations, the only reason why I can think that Mauville has changed so much is because Game Freak plans on altering its role from the original games.
(Either that, or it's all one giant "building" with everything placed inside...it's really difficult to see with the map that was officially uploaded.
Some people bring up the fact that the Game Corner might have to be removed, but it was only the size of a Gym, so it could easily be replaced with something new--nothing that warranted such a drastic change.)