To me, the single biggest opportunity is performance. There's a lot that can be done if you optimize for that: plumb incremental computation through the pipeline from the app logic all the way to the GPU, run your logic in multiple threads, and just generally minimize the work done. I gave a talk[1] which goes into more detail.
Now, I freely admit that many applications don't need that much performance. But I think having a toolkit that is optimized along those lines could be a good basis for doing work on the other things (developer experience) that make a toolkit great.