My personal favourite is Historical. Within that genre, ancients and medieval (not just Western Europe, but also Rus/Eastern Europe and Arabs (Crusades, invasion of Europe (El Cid era Reconquista, Charles Martel). Peasant levies. Some things that have not been done in plastic for 28mm.
I do like science fiction, as well. Specifically more "Starship Troopers" type of sci fi - as well as the apocalypse sets you guys have already done.
I also used to like fantasy. I do not, so much, anymore because of the just plain silly elements that have been included (all the good stuff has been done, so to try to get an "edge" on all the over-done-to-death elements we will get just plain stupid and/or inappropriate to attempt to appeal to the perceived immature nature of gamers as well as shock value).
I do really like the WGF skeletons and Orcs, though. Fantasy with historical elements would be interesting (i.e. Greek Mythology). As well, general hard fantasy troops such as (decently done!) Dwarves, Humans, halflings, and Elves. Perhaps a redone Orcs set and a Goblin set(?).
For Humans, perhaps an "adventurers" series - one of male and the other of female - like the apocalypse sets.
Perhaps a generic "adventurers" series, as well - one that has elves, humans, dwarves, halflings in it - then you can choose from a variety of arms (holding weapons) and heads? Again, one set of men and one of women.
Perhaps mounted adventurers too.
Fantasy cavalry sets (humans, elves, dwarves, halflings (on your ponies already made - just a new saddle - but one that is usable for other figures as well so you can sell sprues of saddles to fit on your ponies so they would be good for medieval figures (Welsh/Highlands ponies as well as smaller horses used throughout Briton and by levies of other countries (yes, I am thinking of Saga)).