This is a re-upload/update of the previous list:
Ideas for improvement:

1) A map with all the campaign bosses packed in it.

In PL for example there's the Nuri's Funhouse.

It would be a great addition.

2) More hairstyles. (3 available)

It's 3018 in SL, come on.

3) A combo enhancer bought only with CREDITS

4) New maps/campaigns/higher level cap. (yeah, this one you expected)

5) Other AoE weapons

The napalm carbine is classified as the #1 weapon in the entire game, since it has AoE damage.
How about some other weapons that can hit 4 targets at a time?

6) Daily reward(s)

A daily reward would feel awesome for everybody who loves the game and plays everyday.
(Maybe a free mod or some credits)
Only SL doesn't include one out of all the Legacy games...

7) More emotes

That's it for now,