Love playing with companions, would love to see more mechanics, systems and even gameplay oriented around them:
1. A more involved companion skill system. This could be simple skill trees for companions (even 5 bare-bones trees for the arch-types similar to WoW Hunter Pet trees would be nice).
Examples: Do you want your melee tank comp to lean more toward dmg? Survivability? Lower the cooldown on your healing comps main heal or give their dmg abilities an aoe component?
Choices could be fun and hopefully allow for more synergy between characters and their companions.
2. A Unify Colors option for Companion gear
3. Either allow companions to ignore class restrictions on armor or perhaps give certain companions class status. Skadge and Torian are bounty hunters, Lieutenant Price is a trooper, etc.
4. More 2player dailies that can be soloed with a companion. More 4player dailies that can be duoed.