One niche market that I’d like to see more growth in is the high-powered sub-5″ screen Android phone market. Currently, the Xperia X Compact and its predecessors are the main players in this market, which also includes some other older phones like the S5 Mini.
I think in general it’s an untapped market, however. Apple’s iPhone SE has proved popular–for people who have smaller hands or who want a phone that can easily fit in their pocket, phones in the 4-5″ range are ideal. But 5, 5.5, and 5.7″ displays seem to be the norm for a lot of premium handsets–it would be nice to see more premium phones in a smaller form-factor.