This question tells me you have too much freetime :P
Seriously, I think many of your controls still have minor bugs like Infra and DX has too using them everyday...
I would focus on your existing controls to bring them into a very stable state.
Then I would complete the documentation.
Only then I would reach out for new things whatever they are.
My favorites are a new wpf richtextbox which screams for speed... Most people do not care for a FlowDocument. They just want RTF not more.
Another fav is a pivot control yeah I know thats a huge thing I know the DX XPF source code which comprises all your extended toolkit sourcecode.
Well just if you feel bored... ;-)
But what I like most about open source controls is to learn from them :-)