How about a weapon which instead of knock back does knock forward? the missile fires over the enemy, landing behind to create a forward blast effect.
How about indication of how many players which are online are actually searching for or in multiplayer matches? For example, adding a second line to the display graph of players. Particularly useful at higher levels where (I believe) its harder to get a match. No point searching if the other players are vs computer.
How about an (toggle on/off) option to automatically search for a pvp opponent at the end of each vs computer match before going onto the next computer match?
EDIT to add
I forgot this one - a store option to buy a premium account for a shorter period of time, at an increased per-day cost of course. I was thinking of a week pass and a weekend pass.
I'd price a week pass at about 1/3 of a month pass, and a weekend pass at about 1/4 - so paying for each weekend in a month costs the same as paying for a month.
This would work for some people who find the monthly amount expensive, and would be a great option for a school holiday - or parents who want to occupy the kids for a week break from school