I actually think good capstones are kind of bad design, in a weird way. They take really good/strong abilities a class could have, and put it in a place where players will only get it on the very last level they can possibly play, and only briefly. So they occupy design space in a bad way and warp the design of character classes. You should probably start getting powerful abilities before you get to the last few sessions you're ever likely to play a PC. On top of that, if they're extremely powerful, the essentially suggest 20 is the last possible level, which I don't think is great.
My experience is that L20 is almost never reached, and when it is reached, played extremely briefly (single-digit sessions), so the ability there is kind of a lie. I'd actually rather see it be a slightly mediocre ability, so players aren't obsessed with reaching L20, than a hugely dominant one.
If one is determined to go for powerful abilities at Level 20, it might be worth looking at the top-tier abilities in the RPG Heart. I wouldn't suggest replicating them (as many are fatal), but they are "go out with a bang" abilities, and I think it might be cool to have L20 abilities be similar, abilities that are really wild, but you only get to use once (or a few times at most), as befits their "end of campaign" status. The one-off design would also make it easier for PCs to remain in the campaign world. Also, if you have a one-off (or few-off) ability and do later at levels 21-30 to characters (or more), then you're not having to design around some broken OP ability, because it's likely been used up.