I have the nucleus of a Dark Age army I have always wanted to expand on. 26 Norman Knights and 8 crossbowmen I traded for back in the day. The Normans fought in many theatres including the Crusades, Sicily, Spain (El Cid),France and Britain. If I fleshed them out, I could get a Saracen army army to oppose them for Crusade battles or El Cid type battles. I have always wanted an Arab/Turkish army.
For me, collecting and painting is often inspired by rules. I usually build armies to fit an army list for my favorite . Our group has been playing Sword & Spear and Command & Colors Ancients. Basing for these games would also work for Impetus.
ECW is strong contender. I have 400 models of 28mm Royalists from Old Glory. They are nice figures and sport some of my best painting. I got them as part of a group project but the group has since disbanded. Now I need another army for the Royalists to fight against. Ideally, I can find a rule set that will allow me to building a second army using some of the figures from the first army. There is a new set out called Baroque from the makers of Impetus. Pike and Shotte units will need 18 figures and Cavalry units will need 9 figures. I could make two armies with my current collection plus 50 Cavalry and 30 Shotte figures. Not too big a commitment for painting. I would need to rebase my current collection which would be a big job as well.
Another period to consider is late Medieval and War of the Roses. I love the look of barded knights with livery and flags, close ordered archers in livery and spearmen in support. This would be a start from scratch project and would require a lot of figures. I could build them for Sword and Spear and this would reduce the figure count. Fireforge and Perry make very nice figures in plastic and I suspect I would break with tradition of metal and go with these fine models.