It seems to me that WA's existing and upcoming fantasy factions seem to overlap well with this era: Halflings, Goblins, Landsnecht Ogres,Trolls...
With that in mind, I'm doing my best to "sell" the Renaissance as an alternative to the Dark Ages for WA's "Classic Fantasy" setting:
For fantasy gamers, it occurs to me that the conquistador weapons and armor look pretty good alongside WA’s halfling militia and what we’ve seen so far of the landsknecht ogres, not to mention some of the gun options for the lizard men seem like a great fit alongside all of the above, and the conquistadors would make a fine fantasy human army, in a style refreshingly different from the medieval fantasy cliche!;
The “Age of Discovery” seems to me to lend itself much better to fantasy world-building anyway: exploration, adventure, and conflict in strange “New Worlds” against a backdrop of Renaissance science and invention on one hand (see the wildly imaginative drawings of aircraft and war machines made by Leonado Da Vinci!), and inquisitions and witch-trials on the other, with an apocalyptic Hundred Years’ War and a devastating worldwide plague as centerpieces to the era, makes for some great fantasy world-building material.;
Hopefully WA keeps making their fantasy figures in more or less similar styles (some halfling handgunners or goblin musketeers, perhaps?) Dwarves and Elves are wide open for something refreshingly different.;
Hopefully we also see more historical figures from this period to go along with the conquistadors and Aztecs… Musketeers of the Guard/Cavaliers, Protestant Puritans/New Model Army, and Corsairs/Buccaneers seem like splendid follow-up historical sets for roughly the same era!
So, Musketeers/Cavaliers, Puritans/New Model Army, and Corsairs/Buccaneers seem to me like great contemporary historical follow-up sets that could cross-over well with the Classic Fantasy line.
And from the suggestions above me, I think Incas and other New World nations and peoples would be a fantastic addition to this line, as would Vlads' armies and the Ottomans, and Golden Age pirates - fantasy gamers would, I think, like those subjects at least as much as historical gamers would!
And I for one would certainly like to see other continents explored - the Renaissance/Age of Discovery would have seen Christian and Muslim empires blundering into all sorts of places besides the Americas for exploration and exploitation, and Sub-Saharan Africa, the Middle East, and Asia seem like they surely have tales of rarely-explored conflicts to tell.
I know almost nothing about the contemporary history of, say, Africa, but a quick glance reveals that this era saw the rise of a West-African Songhai Empire, which was subseqently invaded by a North-African Saadi Empire, a conquest which would have repercussions that would affect the entire world even into the Americas well into the 21st century.
This may not be a popular era for wargaming in and of itself, but the Saadi Dynasty, for example, is surely representative of the North-African armies that occupied Moorish Spain until the late 1400s, perhaps making for a quick-and-dirty conflict with the "Conquistadores" for the later years of the Reconquista, and surely an army suitable for the Saadi Dynasty would cover a span of Muslim history and geography covering a large part of North-Africa and even the middle-east throughout the Renaissance/Age of Discovery, and beyond when kit-bashed with, say, the WA Afghans, or even WWI and WWII European army bits, for those wars' North African theaters, for example....
Come to think of it, a generic North African army set might well fit just as comfortably into into the "Imperial Conquests" line as it does a "Renaissance" line - and, I'll be curious to see how suitable some of the upcoming Chinese and Indian sets might be for representing those nations in the Renaissance era (one suspects that some costumes, tactics, and weapons technologies in general didn't change very much over time - at least, not enough that the casual western observer might notice: when I look up art depicting Renaissance-era armies from around the world, I'm often surprised how modern they look to my eye!)