Quote Originally Posted by Seerow View Post
The main argument in favor of a big capstone is that it discourages multiclassing. No matter what you're doing, there's probably a prestige class out there that does what you're trying to do with the base class, that has some really cool unique features you can pick up by going into it.

By introducing a lategame capstone, you introduce a real tradeoff for multiclassing. Also, as you mention, so few games continue past 20, having a really cool gamebreaking capstone is largely irrelevant, so you can make that capstone exceedingly good and cool, without worrying too much about balance (and even if you are worried about balance, compare it to some of the stuff you get from multiclassing out and you figure 'eh, this really isn't that over the top afterall).
While I've heard the capstone logic before, I disagree with it. The best way to discourage multi-classing is to have cool features that are dependent on your class level to improve. I mean, if you want to throw a capstone on, that's fine, but don't make it distinct from every other ability you've gained. Make it a natural extension of a previously existing class feature series. If you're going to have a capstone, Eternal Rest is a pretty good one. Powerful in a narrative sense as much as a gameplay sense, and a fitting growth to earlier existing powers.