Hard to say.
NF will remain as for sure a solid option, whether or not it is BiS cov for pvp.
The only contender will likely be Necrolord.
When we can equip a regular ability leggo and a cov leggo, as Necro you will get a leggo that proc’s Deathborne. This has the potential to be really strong for double wizard comps, and possibly even comps like WMD, RMP, WWMD, etc. Deathborne gives you a 15% increased spell damage AND makes Fireball cleave, hitting up to 3 people. If you are spec’d into Pyrokinesis and Kindling, you could be getting up to 9 seconds of Combustion CD taken off per Fireball cast. Necro is already pretty powerful when you use Deathborne’s on-use ability giving you 9 sec. per CDR for Combustion and the 15% spell damage. Now you would be adding on the necro legendary for Deathborne making it a proc for 8 seconds, giving Fire a lot more sustained damage and not so reliant on CDs, and simultaneously lowering CDR on Combustion.
There really is no way to know at the moment what will happen. All I can tell you is that its shaping up to be fun for all Fire and Frost Mages next patch getting to test out between those 2 main covenants in PvP. I know I’m certainly excited to see what happens.