TBH I'm not sure the game will be that popular this time next year. Most games only have a limited lifespan before they're forgotten and everyone has moved on to the new best thing. I'm still enjoying it for now, but at max level it can be boring doing essentially nothing and there's only so many times you can redesign, not to mention it can be incredibly tedious.
Nonetheless there's still plenty they can do. I doubt they'd take away anything we've got so far. They introduced Kang on his own so they could easily introduce Kodos on her own next Halloween, and those of us who got Kang (not me, which I regret!) will have the pair. There could be an Ultrahouse 3000 facade for the simpsons house. A Rigelian (Kang and Kodos species) costume for Bart from when the parodied Avatar. As for taskless, npc characters/creatures they could give us Snake's toupee that makes Homer crazy in one of the Treehouse of Horror episodes. Other ideas include a Jack-in-the-box Homer, a donut-head Homer, Bart-fly, teleport machines, Bart's twin Hugo, a facade for Lard Lads.
Basically there is plenty they can add in without taking away anything from us. just like all the other limited time items, those who came late to the game just won't be able to get them.