... Some type of orc cavalry would be cool--right now the only option there is the Banebreak Rider from Night Below, which is awesome but expensive and somewhat specialized. And if the set had wyvern-riders, I would be all over that.
Just a side thought, regarding mounted minis: What about separating the mounts and riders into different packages? Orc Calvary set: just riders. Then pick up the mounts you want for them: horses, boars, wyverns - each a separate (or mixed) set. All riders/mounts using the same inter-lock system. This could also go with a "Leveled PC" set (mentioned somewhere). Fighter set: lvl 1, lvl 5, lvl 10, lvl 20 + same models in rider form. (okay, maybe not

)
In any case, themed encounter sets seem like the next step. What about randomizing just one "Rare" mini in each set:
.. Orc set: includes two spearmen, 2 axes, 2 bowmen + one random leader: (Shaman, high level fighter, chief, drummer, etc.)
.. Undead set: 2 zombies, two skeletons, 2 ghouls + one high level (lich, vampire, ghost, evil cleric, necromancer, death knight)
.. etc.
I have several hundred minis, but I still keep an eye out for different sculps and holes in my collection. I have been mostly in the singles secondary market the last 2-3 sets. However, I would buy themed (mostly visible) sets (if the quality can stay up -- I can get plenty of cheap, low quality stand-ins but there is just nothing like having the perfect piece for your character or encounter).