I’m equally hopeful that we rotate down to only 2-3 all faction big boxes instead of the 5 (including TD) we have now. I’m hopeful that after R&R we get a best of like you suggested which saves a portion of the cards from the current deluxes and introduces rotation of big boxes. That said, I think even the “saved cards” box should rotate eventually.
As per saving the mini’s in it I think it’s a bad idea to do it in a deluxe. They were half of a deluxe, and that would leave a very small amount of cards spots left to reprint older deluxe cards in.
That said @Kesterer had a great idea in another thread (link to that post) of saving mini’s through a new campaign box which has only the mini-faction cards from D&D as tournament legal cards. The mini-factions have such a rich lore surrounding them there’d be plenty of narrative to explore in a new campaign box. Alternatively, FFG could just print a data pack with those cards (20 unique mini faction cards + IDs) in it and sell it for the standard $15 for new players that want it post rotation of D&D.