This could be even automatically enabled, or at least as option.
Example, I own a game in Epic Games, it's in my wishlist, but currently there's offer for Steam at the "Wishlist - best deals" site, which is unusable for me.
In alert filter I have just 1 store filtered already, also for the base game I have a launcher "Epic Games" in "Owned" dialog, so it could be used somehow.
BTW: Text "Warning - This DLC requires base game" could be more accurately, text "for appropriate platform" or "for the same platform" could be added.
There's a warning in the eshops, but such text is missing there too - or the warning is missing completely - in Steam.