I don't think Austria-Hungary would be a good idea. If we separate them to two civs, it is a lot better.
For Hungary my proposal is this:
Hungary
Leader:
#1 Árpád, the man who led the Hungarians to the Carpathian Basin. He would fit for the scenario proposal #2
#2 Saint Stephen, our first Catholic king
#3 Lajos Kossuth, Hungarian statesman, and leader of the 1848/49 Independency War. He would fit for the scenario proposal #1
#4 Miklós Horthy, was the Regent of the Kingdom of Hungary during the interwar years and throughout most of World War II
UU: Hussar, replaces Cavalry but gets +1 movement or +1 strength
UU2: No idea

I don't think it'd need any other UU
UB: Racecourse: Replaces circus, but gives one more additional happiness and horse is requirement.
or
Unique Improvement (UI): Végvár, replaces Fort, but they can be used as roads or if they're in chain, (they act as the Moai) they give +1 culture / tile.
Unique Ability: for this, I have more proposals.
#1 The Arrows of the Magyars:
+1 range and +1 strength for all ranged units.
#2 Cum deo pro Patria et Libertate!
Longer golden ages (+33%?), and units are stronger in Golden Ages (+33%?)
#3 Vitam et sanguinem pro rege nostro!
+15% strength bonus for each adjacent unit and/or +100% strength for nearby great general
For scenario:
1# If they would create a double DLC pack, Hungary and Austria, they could make a Hungarian Independence scenario about Hungary's Independency war in 1848/49.
2# Other proposal can be the Hungarian Migration Period, when we under the lead of Árpád came into the Carpathian Basin and "colonized" it.
What do you think?