One thing I've learned from reading manga is that it doesn't really matter how OP you make your characters as long as you pick antagonists who are actually challenging to them (in their own way).

The mistake Dragonball Z made was to just keep whipping enemies out of nowhere, to the point where it felt ridiculous. You might want to check out Nanatsu No Taizai or Toriko (the latter is kind of cheesy)(okay, super cheesy) for a series where the characters begin ridiculously powerful but you still get that sense of tension. Another example is the Secret Histories series. You've got the Droods with their godly golden armour and the MC gets a gun that never misses and can't run out of ammo... but there's still tension, because there are antagonists who are powerful enough to circumvent these (i.e. doesn't matter if your gun can't miss or run out of bullets if it can't put down the werewolf/infernal creature/other-dimensional monstrosity you're fighting.)

So I don't feel it's so much a matter of /limits/ as a matter of relative threat. You just have to be more inventive when coming up with enemies, though it's good to hint at potential enemies from the get-go so it doesn't feel like you're pulling these things out of your ass 

The manga One Piece does this well -- we get the major factions (and World Powers) from the start, and when badasses show up during the course of the series it really doesn't feel like an asspull, because we already knew that all these powerful people wayyy out of the heroes' league exist out there (even if we'd forgotten).

Just draw them out as needed