A tabletop RPG doesn't have a short loop. Its loop runs on the order of years. Even a regular D&D group doesn't run 1-20 in one session. Heck, we're lucky if any group gets through 1-20 before breaking apart due to real life constraints. XP and character advancement are highly dependent on DM whims, and DMs tend to be conservative when awarding XP due to perceived "sweet spots" or trepidation for high-level play (which experience with pre-4e games would bear out).
I think there's, while not exactly a catch-22, a bit of a contradiction between what people say what they want and what experience has borne out in terms of what they purchase. There's a certain cachet to long term games, and to playing games that take place over years, and I don't think the market is necessarily ready for a game that doesn't promote that as an explicit goal. But at the same time, people's actual campaigns rarely last longer than a year, according to most of the marketing surveys we've seen, and books that sell new crunch, new options for play, are always the best sellers.
I'm wondering if there's an issue with the current D&D setup where new powers and abilities are almost automatically presented as learned, integral to the character, and
persistent with the character. Maybe a better model is a system where picking a class is merely a starting template, which gives you a few background abilities and a few combat options. Then the rest of the character's progression is tied to the acquistion of magic items, primarily consumable, and other boons earned through play. With enough experience, you might also earn stronger uses of your primary abilities.
I think the idea of selling adventures as almost like expansion packs makes a ton of sense. The closest example I can think of is actually 4e's Neverwinter campaign setting. While it still had races and classes, most of the heavy lifting of tying the characters to the adventure was done by the choice of the theme, which all had a story tied into the adventure, and the adventure was meant to be a limited sandbox, only going up to level 10. I could totally see an adventure for 5e being released every year that was pre-packaged with 8-10 new subclasses specifically designed for the adventure, and with 1 or 2 new races as well, and a whole slew of new magic items. This provides new crunch for people who want it, gives a starting point for new players every year, while not overwhelming people who want to play the traditional long-term campaign model.
This also reminds me of some conceptual space that TTRPGs would be wise to borrow from: MOBAs. I've been playing Heroes of the Storm recently, and I was thinking how many similarities to D&D it had. You play in a group of 5 people, all with radically different abilities, who are expected to work together to gain experience and learn new abilities by doing so. I can't help thinking MOBA style D&D would work very well with the idea of limited space adventures. The adventure gives you a bunch of different character types, all of whom have 2-3 unique abilities. You can customize them in appearance and backstory, and maybe with some mechanics borrowed from earlier releases. The characters obtain lots of different potions, wands, and scrolls during the adventure, and occasionally gain a powerful permanent item that's fairly character defining. Leveling gets you more hit points, and sometimes one of your character based abilities becomes more powerful, but that's pretty much it in terms of personal power advancement.
So yea, pretty much some sort of unholy love child of
Talisman and
League of Legends, where you retire your character after you finish the adventure pack and go to the new one. If you really like the characters, the DMG has guidelines to keep the game going.