I only really care about Scotland and Byzantium myself. Here's what I'd like to see:

Scotland:
Scottish Pikemen
Mosstroopers (light cavalry)
Hebridean Pirates
Highland Clansmen (no tartan, please!)

Byzantium:
Kontaratoi (absolutely agree with the point about spearmen)
Menavliatoi (still agree with the point about spearmen)
Tzangratoi (Byzantines did have crossbowmen eventually, just not very good ones)
Trebizond archers that are only recruitable in Trebizond
Any sort of advanced units at all (I mean, if you change history so that the Byzantines reign supreme once more, you're not going to see the same military decline, are you?)
Some Western mercenaries like Latin Knights

And above all I want to see ACCURATE units! I don't mind about native languages or anything, so 'Byzantine spearmen' are alright by me (the names can be changed), but I do want proper unit rosters.