What I currently have as a homebrew feat is that you get two invocations the first time you take the feat, and one invocation every time thereafter. No requirements. This should, hopefully, balance the feat around taking it once on your way to 20, and then taking it multiple times as epic boons after reaching 20.

An alternate version of the same feat created as a racial feat for an undead race allows you to take specific invocations, even if you don't meet the requirements, such as One with Shadows or Ghostly Gaze (or you can take any invocation you do qualify for). So you could do a generic feat that gives extra invocations, or a flavored feat that gives extra invocations but allows specific invocations without needing to meet the requirements. Which specific invocations you allow can add some flavor to the feat, and it allows you to cherry pick which invocations you think would be okay for a non-warlock to get.

I'd probably suggest if you do make a feat that allows specific invocations without needing to meet the requirements, that you don't allow invocations that are specific to a pact boon. Otherwise, you make that invocation available to warlocks with a different pact boon, thus devaluing that specific pact boon.