I think we need to do more work to demonstrate the advantages of tagged memory to potential adopters. I hope that the cost/benefit ratio will be such that others will adopt it.

As for other improvements, it's difficult to pick out specific projects. Off the top of my head, the work on an efficient out-of-order RISC-V core BOOM (https://github.com/ucb-bar/riscv-boom/) is exciting, peripherals such as USB or memory controllers are valuable, and there's a lot to be done in terms of benchmarking and performance analysis.