A feature I think would be very nice would be full or nearly-full support of the FANSI standard.
FANSI is a MUSH extended character set system, made for mostly PennMUSH, but can work elsewhere. It uses the extra characters in the OEM/Terminal font, and allows shading effects, underlining, and inverse bright backcolor effects, meaning you can achieve the illusion of 256 Colors instead of 16 using the shading characters. In the MUSH where I reside, 8BitMUSH, it is used a bit, and has driven an 'economy' of sorts. People do art work for coins, companies sell FANSI products, and it's generally a nice place to be. I'm thinking about moving to the Macintosh, and such I went on a MUSH Client hunt. A friend who uses Mac commented to me that none of the graphical clients supported FANSI fully. Some could do the font effects, but without underline and inverse back-color, it loses something. Above all, I think it would add alot to your already great software, and I know I and the other Mac users of FANSI-enabled MUSHes, would purchase it.