I propose the start of an intergalactic fight with the Klingons to only introduce the Borg with all parties coming together to stop this threat. I'd love to see the original crew take on an emotionless foe in the Borg. With the increased budget and for the fiftieth anniversary this would be awesome.
That being said I'll settle for an intergalactic struggle with the Klingons. I just wish J.J. was still directing.
The Borg could be used. There is precedent set before the alt timeline (First Contact and ST:Enterprise ep "Regeneration"😉 and with the new timeline being mostly free, they could easily write in a reason why the Borg are in the Alpha Quadrant in the 23rd century.
IMO, they'll probably go with Klingons (or Klingons and some other race) and build off of Into Darkness.
They introduced the Klingons for a reason. After that it'll be the Romulans. Personally I'd rather they steer clear of shit like the Xindi and the Romulans and to a lesser extent the Cardassians(as they had some interesting characters such as Elim Garak and Gul Madred.
I'd like to see John De Lancie as Q again. The Borg and the Dominion are also good options although would take one exceptional piece of writing to bring them in that early.