This has been said before but it's worth repeating I think, it would be great for post-support longevity of the game:
The Purging Monument needs to work on every bosses, both to just fight them again if one wants to, and simply opening up your world to invaders, also, a cool "bounty" mechanic could be introduced where the purging monument would show a little titanite slab icon next to a boss you could revive, if you beat him, you get a slab, and the game randomly picks a new boss to revive that would drop a slab, this could also lead to some more coop summons. (But! Make it so tat the Spear of the Church boss is always guaranteed to drop a slab, to allow this covenant to live)
There needs to be a way to respec infinitely, let us pay some souls to restore the 5 respecs or something along those lines, this would indirectly open up a lot of builds that people otherwise wouldn't even bother trying because of this restriction.
When we beat Soul of Cinder, we get to buy an infinite amount of Twinkling Titanite aswell as Titanite Scale, this is great, however gems are a vital part of build-making aswell, as such, it would be nice if beating, say, Gael, would unlock infinite purchase of all the gems in the game, once again to promote build diversity and just try-outs.
If you're worried about this screwing with the intended progression or difficulty of acquiring slabs, how about gating these rewards to NG+2? This way bosses won't be quite as easy and you have an actual reason to get to NG+2 that isn't +2 rings, which you have very little incentive for these days since you get +3 rings in the DLC at any difficulty (maybe make it so that they become one NG+ difficulty stronger, like bonfire ascetics in DSII)
That's all I got, I think it'd be a great way to give players who stay long after now freedom to replay their favorite bosses with their favorite builds! What do you guys think?