I'm actually making them up as I go. Route 26 and 27 got surprisingly changed a lot. An entirely new section was added to Route 27 (the horizontal section), while a section actually got cut from Route 26 (the vertical section), losing that small pool in the original.>>32488344
The reason they redesigned Mauville was probably because Gen 6's engine is shittier than Gen 3's regarding how much can be loaded at once. There are extra route gates everywhere in ORAS (and admittedly there are some in HGSS too, like the gate between Violet City and Route 36, or the gate between Route 1 and Viridian City), and they probably just redesigned Mauville so they wouldn't have to think about how many gates they'd otherwise need.
As to it being bland, I don't think it's bland to be able to stop at the area and then run in all four directions for a long time without needing to hit a route gate (unless you go west, there's a loading point pretty fast in that direction). I feel like Mauville exists to show off the hardware capabilities of the GBA, and it worked.