The best candidates are all B-Tier at best.
From the Horde Valtrois makes the most sense among the Nightborne, as I can’t see Occuleth taking too great of an interest (and Blizzard does seem to favor elevating more female characters right now, balancing the scales).
From the Alliance, there are a few options.
I think Archmage Y’mera would be a solid candidate. A Lightforged Draenei whom practiced magic back before they fled Argus, meaning her knowledge of magic is more extensive than most, to say nothing of her age. She’s not super high ranked within the Army of the Light, so accepting the responsibilities of a member of the Council of Six feels plausible for her.
Another option would be Magister Umbric. With Alleria stepping into the spotlight more and more, he’s had less and less to do, and despite being the, ‘leader,’ of the Void Elves, has rarely felt as important as Alleria. I think its plausible that he could take that seat on the Council of Six, further solidifying Alleria’s role as the leader of the Ren’dorei.
Mordent Evenshade is another candidate. A Highborne of exceptional knowledge and skill. Although he leads the Highborne among the Kaldorei, with the Night Elves settling down and mages within their ranks feeling more at home, he is more at liberty to pursue other ambitions. The only thing I can see disqualifying him is Night Elf fatigue.
Granted Blizzard might also blindside us with a new character. As we’re moving closer to Midnight and that, ‘reunification of the elven tribes,’ maybe they’ll just whip up some High Elf to take the council seat. Depends how involved Dalaran will end up being. It’d be a bit odd to have some random High Elf representing them in Midnight over Vereesa and the Silver Covenant, but who knows? Maybe Blizzard is interested in a high ranking character to act more in a diplomatic role to smooth things over, given the Silver Covenant’s actions during the Purge of Dalaran.
Plenty of possibilities.