@Nailfox, why is hero levels even a thing? Just make it such that your PVP heroes have to be at your Team Level, period.
I don’t see any downsides at all.
In fact we get so much (unwanted) Hero XP dropping from the sky that surely, we can level up our core PVP teams (only about 10, maybe 15 heroes) to our Team Level.
Plus maybe another 10 (if at all!) for PVE. Nothing objectionable about that at all, is there?
We’re not short of Hero XP to build up 30 heroes, max - I dunno about your case but I have never seen any recent post from any players saying we have insufficient Hero XP.
By giving players leeway about PVP Hero level, it would make it that much easier for players intending to min-max to artificially calibrate (lower) their power levels. There are people running around with level 42 and level 50 9&10 star decks when their team level is 68-70. Just keep em honest and make them play PVP at their team level, full stop.