I would go with a mix of jazz, light latin and small-ensemble swing styles. I'd really like to see an arranger with styles that focused on real-world small groups (as in trios and quartets) instead of trying to sound like some huge band with lots of horns and stuff. I mean really - it just looks a little silly to see someone push a button and suddenly sound like Frank Sinatra's full orchestra - it always has and it always will. All that's needed is some basic backing like drums/percussion, bass and light jazz guitar comps. 50 styles that emulated a small tasteful combo would be excellent.

As to sounds - that's easy: the ultimate General MIDI (GM) collection. GM is still a great concept but so many manufacturers have stocked it with poor quality sounds or ignored it altogether. GM keyboards are compatible with tons of software and existing SMF's but noone has yet created the ultimate GM sound set. Besides GM, I would put serious development into the real-world basic keyboard sounds: acoustic/electric pianos and a quality organ/leslie emulation. Without those, you don't have a sale.