I love the ethnically diverse units, and I don't see any reason why it shouldn't expand into the modern age as well. The reasons for ethnic diversity over the millenia have not changed much until very recently, and there is still a great deal of regionalism that goes on despite a constant (but gradual) trend toward globalization. This is what I would expect of regionalized modern units (only mentioned if different from vanilla game):
Western Europe
Modern tank: Leopard I/II or Challenger
Infantry/AT/SAM: green camo
Fighter: German Me-109
Bomber: German He-111
Modern fighter: Eurofighter Typhoon
Eastern Europe
Tank: Soviet T-34
Modern Tank: Russian T-80/90 (any of em is fine) with green camo
Infantry/AT/SAM: khaki camo with AK-47s
Fighter: Russian Il-2 or Yak-9
Bomber: Russian DB-3
Modern fighter: Russian Mig-29
Stealth bomber: Russian TU-22 Backfire (not really stealth, but looks more appropriate than an American B2!)
Far-East Asia
Tank: Soviet T-34 (dunno if the Chinese actually had em, but if they could have afforded them at the time they would have!)
Modern Tank: Chinese Mark 98
Infantry/AT/SAM: tropical green camo (perhaps a Vietnamese conical hat for a helmet?)
Fighter: Japanese Zero
Bomber: Japanese Aichi Val
Modern fighter: Russian Mig-29 or Chinese J-10
Stealth bomber: Russian Backfire again?
Middle-East Asia (see if you can spot the theme)
Tank: Something British, like a Churchill tank, with desert camo
Modern tank: Soviet T-72 with desert camo
Infantry/AT/SAM: desert camo with towels on head and AK-47s
Fighter: Soviet Yak-5
Bomber: Russian DB-3 with desert camo
Modern fighter: Russian Mig-29 with desert camo
Stealth bomber: Russian Backfire with desert camo
This list could be improved of course. I just think regions play a great deal in what the modern units should look like, not just the ancient and medieval ones.