From me there are a couple of different thing that could make this fun.
magic flier with A in flying and A in both faith and reason can add to in my mind a lacking in flier unit's
A master class for assassin cause I usually have 1 in my team all the time so I don't waste money on keys and it have good utility I feel so a master class would be nice
Like other have put out a master infantry something that can be a mix of Edelgard and Dimitri time skip class with A in lance, sword, and axes and maybe B or B+ in heavy armor
Last thing that would be nice is a master class for your dancer something that give more to it maybe add flying or riding to it or just more utility to it's support option cause I can get behind adding to the dancer class some more.