There have been so many things, I'm happy to see the project alive and well after all these years. The HLSL additional tweaks have been fantastic, and I always love seeing fixes and updates to anything from the 70's, 80's, and early to mid-90's from MAME. The work being done for discrete gaming is awesome.
I am still holding on to the chance that the walk sound under Mario Bros. someday will be balanced better with the rest of the game's effects. It was a long arduous road to get Phoenix sounding ‘right’ several years back, including the realization and discovery that the schematics were incorrect for certain value(s), Mario Bros. may be in the same boat.
From MESS hoping to see some improved Atari console emulation. For the Atari 5200, some staple games like Ms. Pac-Man and Pitfall have problems. Improved Atari 2600 emulation with Haunted House working properly would be great. Atari 7800 emulation has seen some great improvements this year for several titles with slight regressions for a couple (Dig Dug and Xevious most noticeable) that I am hoping will improve.
Regardless, the work still being accomplished across the board is incredible. To look back at emulators like DASArcade and SPARCADE, seeing what MAME (And MESS) have evolved from with the very first MAME back in Feb 1997, is just mind-blowing.