(Update: 11-Dec-2015)
Yay, an update!
Time to completion: I don’t have a crystal ball but thanks for asking.
Exclusivity: none, feel free to tackle whatever you want from this list.

Arcade – Part 1
* Konami: Fix Over Drive, get a first look at GX, fix Daisu-Kiss, fix SexyParo protection issue with background objects not drawing, check Sunset Riders protection on diagonals, …
* Jaleco Mega System 1: Fix Ninja Kazan, understand basics about the protection, merge tilemaps with jalmah.cpp implementation in a single device;
* Sega Model 2, fix colors, lightning and last but not least FIFO comms / issues with CPU communicating;
* Naomi: fix Monkey Ball / La Keyboard / Tetris Kiwamenichi SH-4 crash, should be something related to irqs actually looks more like cache RAM;
* HNG64: add network CPU, expecting to make racing games to work.
* Sega Saturn: Rewrite starting from CD Block (ongoing, see saturn_cdblock GIT branch), then SH-2 internals, SCU, VDP1, VDP2, SMPC.
* Birdie Try: fix the long-standing bug with  when the ball is on air (protection related?). Forget it, needs MCU decapping.
* Sexy Gal / Night Gal Summer: rewrite blitter and understand protection faults in latter;
* Improve the Legionnaire / Raiden 2 COP protection emulation, do HW tests, check in particular the collision detection and the sprite drawing protection. Fix remaining issues with Seibu Cup Soccer, more protected than everything else combined.

(To Be Continued, and merged below)

Console/Computers
Acorn
310 / Acorn Archimedes: currently hangs with a “no keyboard present – autobooting” msg. Could help a bare bones KART device (Keyboard Asynchronous Receiver Transmitter) emulation. Fix multiple CPU core issues;
A7000 (RISC PC): doesn’t boot, hits an unimplemented translation fault on virtual address, could help to port some coding from See See Find Out MAME driver;

Atari:
Jaguar: several games hangs due of communication issues or CPU core bugs, blitter needs some work or even (sic) definitely a rewrite.

Casio
Loopy: skeleton driver, BIOS needs decapping;

Commodore:
Amiga 500: disk interface has an horrendous compatibility percentage; various per-game specific issues;

Epson
QX-10: keyboard UART is currently kludged, needs a working UPD7201 core. Some SWs trips a bunch of uPD7220 bugs/annoying features like zooming;

Funtech Entertainment
Super A’Can: sound interface is completely unemulated, various HW bits not understood, needs HW tests;

NEC
PC-6001: cassette interface is kludged to work as a sort of snapshot, needs MCU decapped. PC-6001SR doesn’t work at all (needs custom memory map). Any software that abuses the HW fails miserably (AX-6 Demo, Tiny Xevious 2 et al)
PC-8801: i8214 device and irq firings needs bug fixes, FDC TC signal isn’t fully understood, system softwarelist is barely tested, presumably needs waitstate support too;
PC-88VA: floppy interface isn’t understood, disks doesn’t boot at all due of it; Understand issue with floppies not loading more than few sectors.
PC-9801: floppy interface isn’t understood, disks doesn’t boot at all due of it;
PC-Engine: video timings are mostly off (rewrite in progress, ask Judge), still some annoying timing issues to be ironed out.

Nintendo
Super Famicom: needs open bus hooked up directly in the CPU core, needs cycle wait state support properly implemented into the CPU core, also check this for a full list of bugs (outdated by now).

Panasonic
3do: Hangs on initial BIOS logo display, presumably due of bare bones i/o emulation of fancy chips used, needs HW tests.

Robotron
A5105 (BIC): uPD7220 emulation trips a fatal bug with the FIFO when screen scrolls down, I/Os are totally unimplemented, test images are non-existant;

Sega
Mega CD: some games hangs because they fails their CD error code checking;
Super 32x: various games hangs/crashes due of communication issues;
Dreamcast: GD-ROM comtroller doesn’t work at all, Largely untested, video emulation is sketchy;

Sharp
MZ-2000: cassette interface is completely unimplemented, various video bits not yet implemented;
X1: Fix keyboard implementation, rewrite video code to use a scanline renderer;
MZ-2500: A bunch of games doesn’t load due of floppy issues;

Sony
SMC-777: bankswitch doesn’t work properly, remove bankswitch hack, several unknown bits. remove Z80 BC register usage from the i/o ports and write custom code in place of it.;