Multiplayer for this game would be silly, sorry.
Why does everyone want everything multiplayer? So strange...
I think a good changes would be:
Expand the skill customization system so we have more choices and are capable of more specialization (instead of ending up pretty much balanced by the end of the game.)
More troop types, of course.
Random/unique items/weapons that drop from particularly difficult enemies.
Include more "surprises" and mysteries; things that are difficult to find and experience unless you go out of your way to find them.
More branches in the game's storyline and geography; "points of no return," and the like, to encourage replay to see what you missed last time.
Okay, thank you, bye!