I agree with you. It would be nice if every new Tempest Trials we could choose one of the old TT rewards as the reward for the new one. That way I could have more merges for Joshua(the boy deserves everything) or Windsweep/Close Defense fodder or Black Knights for merges or Steady Stance fodder, for example.
Or maybe they could make a special TT where you can choose the unit you want and you can get like maybe 5 copies with only one full TT?
Fortunately Panne's not in the game yet, but if they ever release her as a TT reward I'd be kinda bummed in some way since, while I could get my favorite waifu for free, the limited merge potential could also be a problem in the future.
I think they will release more TTs with old units, but that's going to take a lot of time probably.