reverse progression DLC
so usually in AC games DLC is used as a way to give the players extra goodies and items after the main game is done.
What would be more interesting to me is if they were used to show off the later periods of the player character's life, where they lose abilities that were inherent to them from the beginning: basic combat moves like guard-breaks, the ability to climb (basic jumping still intact, but you're stunned for a time when you land from a fairly big drop) reduced speed when hunched over in sneak mode, and in general all the upgrades you've made to your character over the course of the game.
Of course this wouldn't actually permanently affect your main game, it would be a separate thing. And you wouldn't lose everything at once, it would be a story told over many years, a bunch of little vignettes.
This is all in service of giving players who've completed the game a serious challenge, learning how to compensate for the dulling of their formidable claws. A disempowerment fantasy.
That would be really cool, actually.
I can almost see a separate fanbase rising for specifically those sections because they're almost totally opposite to how we think of games today. Yahtzee once wrote in an Extra Punctuation, about something similar. You might like to read it here;
http://www.escapistmagazine.com/articles/view/video-games/columns/extra-...
so you want Revelations to release the Old Altair memory scrolls as DLC instead of being part of the main content? 
edit:
I know that's not what you're saying; I'm just being facetious.
But seriously, we've had a taste of this in the Old Altair memory scrolls and, to a lesser extent, with Old Ezio in Embers. We've also seen it, sort of, in the aging of Haytham Kenway through Forsaken, Rogue and ACIII. Granted, he wasn't playable for his whole life so we don't know how much his abilities were altered, if at all. We also saw aging in NPCs between Black Flag Rogue and ACIII. That said, we again never got to see their aging to completion (save Achilles), and we never got to play and experience the decline ourselves.
I think an "aging to lose abilities" could quickly be countered with "use of more tools." If you can no longer fight your way out of a situation, you'd be more inclined to engage at distance. Surely your trigger finger would still work.
“Force has no place where there is need of skill." Herodotus
I think an "aging to lose abilities" could quickly be countered with "use of more tools." If you can no longer fight your way out of a situation, you'd be more inclined to engage at distance. Surely your trigger finger would still work.
That's basically how they did it in Revelations. Ezio wasn't supposed to climb too well anymore due to being old, ergo hookblade. He could still climb just as well as he could in AC2, of course, so it was more of a narrative thing than a mechanical thing. But yeah!
re: just using guns, obviously part of the conceit would be that you still need to be sneaky and move through close quarters situations, and also that you're the only one who can handle it. (Though of course there's no reason your abilities with guns and other tools could not degrade just the same) And I'm definitely not just talking about the more aesthetic depictions of age (Ezio, Haytham), or age being used as an excuse for slow-walking sections with little real gameplay. (Old Altair sections)
I'm talking about the loss or altering of abilities being legitimately systematized, so you have to think about the way you play the game differently. "I can no longer land from drops quietly and I'll be stunned for a few moments when I hit the ground, so I need to make sure no enemies are around or I'm dead" or "I only have the ability to defeat the most basic rank of guards in combat, I have to make sure I don't fight any more experienced enemies" or "I can't climb, I can only get up buildings using ladders, so I need to sneak past this guard set-up solely on the ground to get to the top of this roof".
I'm talking about the loss or altering of abilities being legitimately systematized, so you have to think about the way you play the game differently. "I can no longer land from drops quietly and I'll be stunned for a few moments when I hit the ground, so I need to make sure no enemies are around or I'm dead" or "I only have the ability to defeat the most basic rank of guards in combat, I have to make sure I don't fight any more experienced enemies" or "I can't climb, I can only get up buildings using ladders, so I need to sneak past this guard set-up solely on the ground to get to the top of this roof".
The reason I like this idea is because it introduces way more consequence to Assassin's Creed than the series has had so far. Strong consequence is what really makes a game feel more like a game and less like a toy.