I think you've got most of it there - this era overlaps the Napoleonic era on the earlier end, and WWI on the later end, and you can surely find a use for at least some of the more flexible Napoleonic and WWI era sets.
Don't be afraid to check out the WWII sets as well, there's a few of those that are flexible enough to overlap the later end of WWI, and would surely fit in alright in the latter part of the Imperial Conquests era - the Italians and French especially come to mind.
You might need to handwave a couple anachronisms here and there, but if you and your gaming group are OK with that, then that would probably cover most of the Wargames Atlantic historical territory. You might spot a couple digital sets I wouldn't think of outside of these rnages (for example, the Victorian Era, the Wild West, and the American Civil War would all be contemporary with much of the Imperial Conquests range), and so digital is probably worth looking at in more detail, but there's surely not a lot left in boxed plastic sets.
As usual, things are a little more flexible for you if you're into fantasy gaming - and the Imperial Conquests equivalents to fantasy would include things like Victorian Sci-Fi, Steampunk, and Pulp adventure, horror, and sci-fi. If that's a direction your interested in, then be sure to check into Wargames Atlantic's Death Fields, Classic Fantasy, and Pulp Adventure ranges (the Lizard Men and Bulldogs for example would fit in well with Victorian sci-fi), as well as the new "The Last War" tabletop wargame available through WA (which runs by default on WWI figures but doesn't necessarily have to and could easily be adjusted to a pulp Imperial Conquest setting), and don't be afraid to check out earlier eras for armies to populate lost continents with (WA's First Empires Persians - perhaps combined with bits from the Death Fields Accessory Sprue - would make fine Atlanteans or Lemurians, the WA Renaissance Aztecs would look right at home in any Hollow Earth or Lost World setting, the French Resistance can be modified into pulp adventurers, gangsters, Cthulhu investigators, and other such characters pretty easily, while Digital Atlantic has lots of stuff for pulp gamers to work with, too!)
But if you're strictly going for historical gaming, and you don't mind some anachronisms here and there, the only thing I can think of to add to what you've already mentioned would be to check out those WWII French and Italians, and scan through Digital Atlantic to see if there are any surprises there.