"Warlock is a dip class, yadda yadda yadda"
How about this change:
When you gain a new level in the Warlock class, you can swap all invocations and choose new ones.

It would make invocations work more as "prepared spells" (though with a lower speed of change) and less as "known spells". It would allow for Warlock players to experiment more with all the different invocations (the limited number of invocations is a problem that's compounded every time Wizards release new invocations). It would encourage people staying as Warlocks instead of just using them as dips.

It's a definite upgrade; but is it more of a quality-of-life upgrade (less stressing about which invocation to choose when, more fun with the different invocations), or is it a munchkin upgrade?