This is a list of general, non-driver specific things, that need improvement in MESS. If you want to contribute but don't know what to do, this list can give you some pointers.
Update this list! Many of the things on this TODO list are actually way out of date and have already been done.
Unify the Motorola 6845 implementations. Currently, we have two implementations in MESS, and one in MAME. It would be nice if the MESS drivers using one our implementations could be ported over to the MAME version. (Only one more to go!)
Clean up old drivers. There are still lots of drivers without an active maintainer and as such, they are sometimes in a very bad shape. Converting them to current coding conventions and adding comments would be very useful.
Improve skeleton drivers. In MESS, there are many drivers in a very preliminary state. These are usually added to the source to document the available BIOS dumps and to provide a basic description of the hardware. Adding more functionalities, with the final goal of proper emulation, would be very appreciated.
Implement mizapf's XML format to store ROM infos in more drivers, to help MESS loads cartridges with multiple ROMs. Currently, we use a zip file containing the ROMs and a layout.xml file to describe them (where to load, how to handle them, etc.). However, some systems may need subtle modifications to make use of this system.
Remove dependency on mslegacy.h from all drivers. This files includes functions and macros that were removed from MAME, but not yet from MESS.