Well, that should be a thing.
Originally the "inheritable quirk" and the "growing strength quirk" were not the same. One was given by AfO and the other belonged to the original user, his brother. Both merged allowing the strength quirk to also be passed along with the other… if the inheritable quirk could merge with the other (it is this merged result we call One for All), it wouldn't be unreasonable if it could merge with others quirks as well as it kept being passed on.
When you think about it, it would be the logical consequence. The "inheritable quirk" had no ability other than passing itself down. It makes no sense for a quirk pass down the ability of it being passed down and that's it, unless it can also pass other things along with it. It seems the original purpose of this quirk is merge with other quirks and allow them to be passed along with it, creating successors that can have access to more and more quirks.
Those other quirks should not be getting continuously more powerful, though, as "getting more powerful with time" was a characteristic of the strength quirk alone (that reached absurd levels since the inheritable quirk allowed it to keep strengthening through many life times). Unless, of course, that characteristic would also merge with the other quirks...