You might try a box of Jumpstart packs. Each pack contains a group of 20 cards that are intended to work as half a deck. You open two packs each, shuffle them together, and off you go. I believe each pack comes with a list of the cards it contained, so once the games are done, you can split the deck back into its component parts. Put each half-deck into a box together, and any time you want to play, you can each pick two half-decks and play. I believe this gives you 276 possible decks you can create from just one box (and of course a second box increases that number), and thousands of games where you and your opponent never play the same two decks against each other. Even if you just open all 24 packs and choose which ones go together, and keep them together forever, you still have 12 different decks to play.
This gives you a ton of prebuilt decks that are all mostly balanced against each other (with duel decks you'll find that while the two released together may be optimized to play against each other, playing one from 2012 against one from 2016 may give you an unbalanced game). And if they ever release a Jumpstart 2, presumably you'll be able to expand your half-deck collection to give you even more variety.