If I wanted to play a fiend, I would be fine with how Tieflings are presented to us since 4e, because its literally a human in a Devil costume.
That's my point. If they want to give the players the ability to play a fiend, then go all the way. Let them play fiends, including their powerful abilities and maybe a level adjustment.
If you want to have humans with demonic blood keep it as humans with little oddities.
