One of the major sticking points for fantasy and rpg mods in Civ4 is the inability to create multiple maps for dungeons, planes planets and shops that could not be displayed on the overland map.
It'd be extremely handy if, in Elemental, it was possible to tie all sorts of maps to one another, in a variety of ways: a daisy chain, overland/dungeons, land/sea/air.
It'd also be nice if those off-maps could be randomly generated using scripts that differed from the script used to create some or all of the other maps -- say, if I wanted to use one script for the static overland map, and several different scripts for randomly generated dungeon maps accessible from the overland map.
And it'd be even better if these dungeon maps could be randomly created throughout the play session through scripts, rather than just generated at the start and left alone.
Also, it'd do wonders for the sci-fi community if you could create multiple levels of maps -- say, galaxy, individual systems, individual planets within those systems, and individual locations within those planets -- where some maps were randomized (say the systems) while other lower levels were static (say the planets) and had all their entrance/exit locations placed automatically and appropriately by script.
And it'd be even more amazing if you could have an interface that allowed the modder to set entrance restrictions, and give the player an interface that allowed them to select which and how many units they'd like to bring in/out of the various maps.
Hopefully that's not too complex to include, as it's been a huge bee in the butt of the Civ4 modding community, and it really would do a ton to convince many of the hard working players over there to switch to Elemental -- because, after all, it's something that Civ4 does not and will never have that is massively called for.