I am making a game with a certain amount of levels. It takes a bit of effort to get them working so I was wondering what systems we have in place for selling DLCs or if I would have to have a different version of the same game. Calling it version 2, or coming up with a sub name. My Game....My Game: Vol 2, etc and then releasing that second app on the store as well
plenty of games go the route of letting a user import more/custom levels through the data disk — if you were so inclined, you could possibly sell "level packs" that users could import into their data folders, and play on just the one game bundle. (personally i'd prefer this tactic over seeing multiple vols. of the same game filling up the store/my game launcher)
this tactic is usually used for puzzle games with deterministic layouts (soko, sketch share solve, sparrow solitaire) but also has uses elsewhere (P-racing user replays, for example). i don't think i've seen an author explicitly create paid DLC using this method though, so you could be the first!
if you're distributing on itch.io, you could probably make separate pages for each content pack and sell them that way (you could also offer some kind of 'bundle deal' that way using itch's sales system).
if you're on catalog, distribution is a lot more limited (every listing has to be a full PDX bundle; no current native support for things like demos or bonus content via data disk) so in that case i'd recommend putting the DLCs up for sale elsewhere, and linking to them from the "More cool info" section of your main game's listing page