Let's say you really like subclass A, but you also really want a specific feature from subclass B. By the rules, you can't take more than one subclass per class, so we'd need to move into the realm of homebrew to make this work. What would be a good mechanism to do this with?

One option I've considered is a feat that give you the features up to 3rd level in a subclass of your choice (since all subclasses are gained at 3rd level or earlier). Optionally, we could allow you to take it more than once, but maybe restrict it to once per class. Also, we could allow you to take the feat again in order to upgrade your second* subclass to the next set of features.
*I say "second", but you wouldn't necessarily need to have levels in that class at all. Of course, some subclasses give bonuses to the core features of that class, so without those core features the subclass does nothing.

Another option might be a "mix'n'match" option, where you can give up all the subclass features gained at a specific level in order to gain the features of a different subclass gained at the same level (or earlier?). Although I feel like this might be prone to abuse, especially with the "or earlier" option, where you trade away a ribbon for a more substantial ability. Not all features are of the same value, and this is generally figured in to the value of the rest of the abilities for that subclass.

An interesting variation on the mix'n'match option might be that you have to pick a different subclass to gain features from every time you get subclass features. As in, you can't gain features from the same subclass twice. You still have the problem of being able to pick the best feature from each subclass, but if a subclass has two nice features gained at different levels, you can't get both of them. Also, probably no "or earlier" clause for this option. I actually kind of like this as a general "custom subclass" option.

As another option, similar to the feat option, once you reach 20th level you could spend epic boons to progress other subclasses. Taking that boon more than once would progress that subclass to the next set of features. You'd always get all the features up to 3rd level the first time, and after that it would depend on what levels that class gets new subclass features, so it would take three or four boons to max a subclass. This option goes hand-in-hand with another houserule where you can spend epic boons on more class levels, but your character level is locked at 20 (you only get the class features, no HP or proficiency bump). The problem with this is that most campaigns don't reach 20th level, or stay there for very long once they do. It's a nice option to have, but not something you could reliably use.

There's also the "gestalt" option, where you just pick two (or more) subclasses and get both sets of features. However, this one differs from all of the above in that it just gives you the features with no cost. It's fine if everyone is using it, but not if only some people are using it.

How do you feel about these options? Would you use any of them? What are some other ways this could be handled?