Yeah, I'd like to experience the 'classic' gameplay (for the first time really, I didn't play GTA3MTA during its heyday) with excellent sync and stability. Portland and Staunton would be easily scripted since they're just the respective islands alone with Ammunations/PnS, random spawnpoints, vehicle/weapon positions and other basic stuff like that, but SSV would take longer due to its many gamemodes and other related complexities. But it'd be fun I think!
Even though the potential of LU is greatly increased compared to MTA's pioneering techniques, I'd still probably aim for a carbon copy of the gamemodes just to preserve their nostalgia somewhat, and enhance them later as separate "resources" with more weapons, spawn menus, fun gamemodes, and a combination of islands.