William Shatner is in consideration to return in the next Star Trek film as an aged James T. Kirk.
Although Shatner's Kirk met his end in Star Trek: Generations (1994), with the new alternate continuum created in the previous film the writers now see an good opportunity to resurrect Kirk.
According to J.J. Abrams, the time travel-alternate reality concept used in the previous film was a deliberate ploy to enable a reboot for new films: "The idea, now that we are in an independent timeline, allows us to use any of the ingredients from the past - or come up with brand-new ones - to make potential stories."
The writers currently feel that the Borg are the most likely candidate to be the antagonists for this film, drawing parallels with the Star Wars theme of "man/machine" immortalized by Darth Vader; they feel it would be interesting to get the Borg, who mechanized their organic bodies but left them without any personality, to rediscover their spirit just as Vader did.
It is also chronological to bring in the Borg from The Next Generation after the Romulans from the original Star Trek.
Interestingly, the writers considered using the "Planet X" storyline, where the Enterprise crew encounter and team up with the X-Men to save the universe!
However, this was ultimately vetoed in view of rights issues, keeping the focus solely Trek-oriented, and the fact that casting Patrick Stewart as Charles Xavier instead of Enterprise captain Jean-Luc Picard would be too much of an outrage among Trek fans