how would you narrow down the list of good 3.5 feats to what should be converted, keeping in mind the general power level of 5e feats (weak but broad with 4 outliers) and not obsolete anything already printed? Having not played 3.5 in a few years now, I'm having trouble remembering the good feats besides uber-charger kits and metamagic shenanigans. I don't think many 5e fans want uber-charging back and metamagic is now a staple feature of sorcerers instead of a group of feats. I'm enjoying feats that are largely not a balance concern outside the few outliers.
You seem to want feats to have no mechanical impact on the game. If so, take Weapon Focus(x weapon that you'll never use). Or take toughness at level 18. You get to feel "tough", right? You have a feat that says so!
5e's balance problems aside (I mean its general power level), the bar for any further iteration of a game (D&D or no) or anything else (think Office or Windows) which wants to "upgrade" has to manage 2 things: A) not leave behind good things and B) make enough good things that it's worth the upgrade pains.
If you're using Windows 10 x32 and you hear that the next build of Windows 10 will be x64 only, then you'll probably not switch because you'll lose easy access to your windows-only 16 bit programs. This is an example of A). Now imagine Microsoft actually built in an on-the-fly 32bit emulator into that x64 build. Now you'll upgrade. This is why WotC should convert their 3e material.
If you know it's going to cause your boot problems and you don't want to spend 4 hours restoring your rig, you won't go from Windows 7 to Windows 10 unless you care about the .vhdx mounting capability. This is an example of B).This is why WotC should release their 3e material for a low/free price.
So what is the bar for what feats should be imported (by WotC, of course)? The same as the above: if it's useful. Does an optimizing player want that feat? If so, it's in. Would we laugh at said feat? Then don't import it. Here are some SRD examples:
Skill focus is in (if truenaming is)
Lightning reflexes isn't worth a feat slot (use an x to y feat instead)
Self-Sufficient is out (the heal and survival skills never need to be pumped)
Widen spell is in (thanks to MM reducers, +3SL for +100%spell area. I understand MM is different in 5e, I'm just comparing based of 3e)
Extra Turning is out (even with a load of turning pools, it's a subpar feat. And that's with extreme optimization!)
Improved Counterspell is out (there are better ways to counterspell than to blow slots of higher level than your enemy)
Greater Spell Penetration is only in if your DM has fixed SR
Leadership is obviously worth it, so much so that it's in need of nerfs
tl;dr 5e doesn't have enough good new toys and it leaves behind too many old ones.