honestly, capstone at 20th level is so irrelevant for that as it is not seen in 99% of the games.
Capstone should be at 11th level with possible improvements or extra usages later.
11th level means it cannot be taken by 2 classes for multiclass character.
now when I think about it, I now remembered that
@DND_Reborn did with his variant:
Have you played a PC to level 20, to obtain your capstone feature and rejoice in newly found power and ability, only to have the adventure... end? Have you played many campaigns, making it to level 10 or 12 even, just to find the adventure is suddenly over, and look at those higher lofty levels...
www.enworld.org
all NEW abilities should come before 11th level, maybe at reduced power, so you can enjoy them in most of campaigns.