Austria. For much of its history, inextricably linked with the Holy Roman Empire which could be a problem with ol' Red Beard as a German Leader.
On the other hand, as a separate entity, has some intriguingly different aspects for a Civ VI inclusion:
Potential Leaders: (my favorites, personal opinions only)
Maria Theresa - the Ultimate Diplomatic Leader, possibly with either extra Envoys or better yet, the ability to use Envoys to improve relations with other Civs as well as City States.
UU: Pandur (English: Pandour) a musket-armed Recon Unit with extra Pillaging capabilities - the originals were Croatian border troops, the firs unit of which was raised at the beginning of the War of Austran Succession in 1740 and they gave Frederick the Great of Prussia fits throughout the midd of the 18th century - no supply wagon was safe in the same province with a Pandur!
Charles V - potentially another Dual Leader, this time for either Austria or Spain, which alone makes him worth considering.
I suggest that his UU as HRE/Austrian Leader could be the Landsknechts, the first of which, after all, were raised by the HRE Emperor Maximilian I in 1486 - not quite contemporary, bu close enough for Civ!
Landsknecht - Pikeman replacement with extra Melee factor (the current Zweihander Promotion for Melee units is utterly wrong: they were the 'double pay men' in Landsknecht pike units that gave them extra capability against other infantry units, not cavalry)
Maxmilian I - more HRE than Austrian, but Habsburg, art patron, tax reformer (although not extremely successful at it), first to use the international banking families to extensively finance his reign (extra Gold from Trade City States or Great Merchants?), married out quite a number of family members for diplomatic gains in all directions, and of course even more definitely connected to the Landsknecht as a UU.
I could see either Austria or the HRE as a Civ having one of the new 'malus' type attributes of any city they conquer or acquire that used to be a City State goes back to being a City State, but with Austrian/HRE Suzereignty, but that makes the Diplomatic Marriage that is an obvious Austrian/HRE Unique much less useful. Perhaps the Marriage Advantage could extend to using a "Marriage" Envoy to instantly and automatically remove another Civ's Suzereignty over a City State - driving Corvinus nuts, but making it more difficult to attack Austria with 'allied' City States.
Bottom line: despite the overlap of language and culture and geography, Austria/HRE is distinct enough from 'Germany' as a separate state/Civ to be a possible inclusion in the game, using a Diplomatic Influence model instead of a Production/Military Model that is more appropriate to Germany proper.