Here goes:
1. Skirmish mode, I love Skirmish modes on RTS's.
2. Heroe characters for each species. Like the Alien Heroe could be a Predalien Queen (Still think Pred Queen is a horrible idea), Marine heroe would be Ripley, and Predator Heroe could be some random Elder Predator, or my favourite is Wolf (If he doesn't die in AVP:R). And for Skirmish mode, major characters from the films for heroes, such as Hicks, Private Hudson, City Hunter, Jungle Hunter, Celtic, Scar, Gill, and I wouldn't be sure how you could have many Alien Heroes.
3. Units to look better, and not like some cartoonish type character, and I agree that aliens should be blacker, and not so light. Marines should look like the designs of the marines on Aliens. Preds were pretty good on AVP: E 1.
4. Cutscenes between levels, not a briefing screne where you have to read alot.
5. Special Species attacks, like for aliens you could have a special swarm attack, where it summons aliens (Warriors) who tunnel out of the ground and then instantly kill all that get in there way, then they dissapear into the ground again. Marines could have an airstrike, where dropships fly over and bombard the battle feild with bombs, and rockets. Preds could have a clan rienforcement attack, where a Predship lands, and soldier Preds blast all the Plasma cannons at enemies, then flies away. (For these you'd need Comm-Tech, Shrine, or Queen)
6. Better story.
7. More units allowed for aliens and marines, Predators should have a bit less than marines since they are so tough.
8. More realistic upgrades, no dumb, fat glands on the side of alien heads!!