I would recommend,
For epic battles with simplistic yet enjoyable empire development try:
Shogun: Total War Gold Edition
Medieval: Total War Gold Edition
Rome: Total War Gold Edition (Good Mods are, Roma Surrectum II, Aristeia, Rise of Persia)
Medieval II: Total War Gold Edition (Good Mods are, Third Age Total War, Rise of Three Kingdoms, De Bello Mundi, Stainless Steel, Broken Crescent)
Total War: Shogun 2 Gold Edition (Good Mods are, Performance Overhaul Mod, Radious Unit Mods, Radious Campaign Mods, Darthmod)
The two best official Star Wars strategy games:
Star Wars Galactic Battlegrounds: Saga
Star Wars Empire At War Gold Pack (Good Mods are, Republic At War, Rise of the Mandalorians, Absolute Corruption Mod)
The Age of Empires Series are good historical builder/fighter RTS:
Age of Empires Gold Edition (Good Mods are, Userpatch)
Age of Empires II Gold Edition (Good Mods are, Age of Chivalry, Tales of Middle Earth, Userpatch, All Roads Lead to Rome, Rome At War)
Age of Mythology Gold Edition (Good Mods are, EPIC TEXTURES MOD, Age Of Mythology Unlimited Population and Build Mod)
Age of Empires III (Good Mods are, Felix Hermansson's Draugur AI)
Check them out. Plus they're all pretty old so they should run pretty fast on most PCs, and for me they are compatible on Windows 8.1.
Last, but not least try Armada 2526: Gold Edition, a space sim, made by the same developers who are making OE.
It is very cheap to buy, so if you don't like it you're not wasting much.
What it lacks in complexity and in-depth detail, is made up for with a good art style, great music, and fun inter-galactic civilization development with lots of unique and interesting races to talk, trade and fight against.
It has a massive and varied tech-tree, ranging from wormholes to battleships to psychic powers! It really is underrated in my opinion, and I can't stop playing it, so I'm even more looking forward to OE!