Personally? New models, hands down. Challenge modes are fun, but I prefer raiding and totally new content rather than different modes of content.
So, it wouldn't be worth it to me to have them go back and do Challenge modes for old content at the cost of new models. A few Challenge modes paired with new content is just fine.
Character models, definitely! Right now it's like 8 bit Mario in a 64 bit world...
New content, new models.
For those who think a certain TV-show "whisperer" is a natural at what he's doing, and still think about dominance when it comes to dog. Basically, people write extensive research spanning over 30+ years off in favor of flashy tv-shows and dramatic scenes. Pathetic.
Given the lack of interest in Challenge Modes by the community, I'd expect the majority to ask for new models and I agree, new models please!