Lv30 (Warframe) - Zephyr - Boltor Prime - Lex Prime - Dakra Prime - Gold Seeker -
Lv40 (Firefall) All Battleframes
Lv60 (Neverwinter) - Rogue - Fighter - Wizard - Guardian - Cleric -
I guess my suggestion is more a best for the players, but that doesn't necessarily mean it would be best for the game.
With that being said, as long as the rewards are scaled I don't personally have a problem with forced downleveling. I mean I enjoy it in GW2, but I wouldn't like something like that in WoW. I don't know whether it's the game styles that make it different or just because I'm used to the method in each game.
Now if you are over 50 then it should force mentor with stat caps based on the content. Still leave you OP, but not super duper OP. For Example:
You enter a L50 T1 Raid, your stats are capped as though you had BiS from L50 T1, in other words, you meet or surpass T2 requirements. It's still gonna be a cake walk for a full L60 group at that point.
L50 T2, capped at BiS T2, ready or surpassing T3, T2 content will still be a cake walk at this point.
L50 T3, capped at BiS T3. This is still going to make ID be semi-challenging, but not too difficult as you still kinda out gear it. It keeps the content fresh and relevant. If you are running with F2Pers, then you are not making it excessively easy for them so they are getting more of a taste what the real raid content is like to make a better informed decision if they want to sub up and move on the L60 raid content.
In other words, it would still remove most of the challenge, while not making it too too easy.