I was very skeptical when I saw LFD Warlocks announced, but the lore explanation and some of the things discussed here have really changed my mind about them. Im actually quite tempted to make one now as a ‘Binder’ or similar, not so warlock sounding, class.
I do wonder; With Light being so strong against Fel, would LFD Warlocks be immune to Fels corruption? I personally think they would and that makes them even more interesting from an RP perspective. Your character could have a 'Only we are capable of wielding the Legions magic safely, all other Warlocks are bad" attitude and really lean into the ‘holier than thou’ vibe of the Lightforged.
Would a fully fel-brimming Eredar be able to become a Light-wielding priest?
It’d be painful, it’d be rare and it wouldn’t be very plausible. But it can be done, surely. I admit I remain a little unconvinced of Blizzard implementing the race/class combo in any meaningful way, but I’d like to be proven wrong (in this case)
I simply pray we’ll get some story beats updating us on how the Light works (or can work), or maybe there will be some new means of manipulating magics of all kinds.
Not that the concept you mentioned doesn’t sound interesting, but the setting doesn’t maintain this “Lightforged thus anti-Fel” lore in any case. At this point LFD warlocks may have been actually pure of intent and soul at the time of their Forge of Aeon ritual and did a complete 180 turn into dark magic over the past in-game decade as the Army of the Light was repurposed, joined the Alliance (which is a mixed bag of moralities, beliefs and schools of magic) and got dunked on by Tiki masks and whatever other losses they suffered on Azeroth.