Why assume that every Civ's military needs MBTs (Main Battle Tanks)?
A non-militaristic Civ with defensible terrain might very well emulate Sweden and produce the "S Tank" -- actually not a tank (turretless) but with a very low profile, relatively low ground pressure, and a decent gun. Relatively inexpensive, not great on the offense (those turrets are really helpful), but possibly very, very good on the defense.
A modern Civ facing a WWII-esque challenge might emulate and update America's WWII decision to build Shermans -- light enough to ship across oceans in quantity -- by building LAVs (Light Armored Vehicles -- wheeled) just as the USMC and the French Foreign Legion have done. Fast and very good on the offense while suffering some on the defense.
Let's also not forget that hovercraft -- Surface Effects Vehicles -- do exist and these, sufficiently armed and armored, might readily quailfy as a "tank" in, oh, say, the Mekong Delta (where I believe they were actually employed -- granted, not as tanks; but it's not really so radical, given that Austria-Hungary tried to build the first SEV in WWI). Of course, this would ideally work with a different terrain/mobility class, but I know we're all dreaming of a decent scenario editor ...
Best,
Oz