Top 10 in order of "cool factor", borrowing from previous years:
1) Support for cross-platform (SDL2) rumble effects for systems like the PSX (dualshock), N64 and PokeMini vs. the basic output handler support that currently exists
2) Model 2 accuracy improvements
3) Macintosh Plus/512 speed fixes to resolve this issue
4) More accurate Cave CV1000B timing
5) BGFX vector bloom
6) Framework for future improvements to BGFX presets (examples include optional auto-assignment of "arcade crt" to arcade systems, "green screen crt" to apple //c and "green screen lcd" to original gameboy)
7) Votrax/SC-01 improvements
8 ) Vectrex artwork improvements
9) More collaboration with standalone emu authors of license-compatible projects to improve MAME drivers for high-profile systems
10) Improvements to high-profile console drivers such as nes (more accurate ppu), pce-cd, atari 5200 and n64 (speed optimizations)
As with previous year wish lists, most of these aren't entirely practical and I'm certainly not expecting anything here anyway, though it's fun to imagine.