I'd like to see new presents (10-15 new to add the variety), biomes, and maybe another one or two characters that can be linked back to the original games if there's enough time to focus on them. I'd be willing to pay for this content as well, but I do feel it's important NOT to split the community up over it (eg: eliminate the potential for "you can't play with people unless you have the DLC").
The way that these menus are set up, you have to scroll up and down to find what you want, which is kind of annoying and will become more annoying if you guys add more stuff in 2.0 and beyond. Scrollin' for days.
It would be so much easier to navigate if they were set up in a grid format, and is more aesthetically pleasing to see how much you've unlocked at a glance.
@Nap: Would pay for extra DLC content :)
Our entire menu system could use a bit of polish