I would also like to use a Romulan angle, though I'd like to cut out the Klingons entirely.
We should not need a common enemy to make new friends; if we do, we can never have lasting peace.
Rather, I would like to see something more along the lines of
Star Trek VI, where the conflict was not primarily about defeating an enemy (though that was certainly part of it), but rather it was about achieving peace by overcoming obstacles intrinsic to the peace process itself, such as a troubled history and destructive/hateful elements and attitudes within the two societies themselves.
The choice: Klingons or Romulans? 
In the TMP era, the Klingons were the natural choice: They were a recognizable species, and our protagonists had a lot of history with Klingons, which conferred a lot of personal biases towards the species as a whole.
For the reboot universe, I think that the Romulans fit this role much better than the Klingons.
It was a Romulan who killed Kirk's father, and destroyed Spock's homeworld, killing his mother and further isolating him from his Vulcan heritage in the process.
Furthermore, the state of Vulcan in Romulus largely defines the separation between the two universes.
In the one universe, Romulus is gone but Vulcan lives on.
In the other, the situation is the reverse.
A tale of two Spocks
Furthermore, let us consider the case of Spock Prime:
He is the only person who has traveled between the two universes and is still alive; the last surviving link between them.
Furthermore, he has three sworn duties:
- Vulcan-Romulan reunification
- Saving Romulus from destruction (in the Prime universe)
- Preserving Vulcan culture (in the alternate universe)
One could see how duties 1) and 3) might be combined. In the alternate universe, they've already gleaned that Romulan and Vulcan culture are connected.
Therefore, reunification could be one way for the Vulcans to reconnect with their heritage.
(However, some might object to this, claiming that assimilating a group of about 10,000 Vulcans into a population of billions of Romulans would result in the dissipation and destruction of Vulcan culture; compare it to the many indigenous languages and cultures that have been destroyed through assimilation here on Earth. So, it's a complex issue, which is as it should be.)
Saving Romulus 
As for duty 2), that of saving Romulus:
In
Star Trek Into Darkness, Spock Prime mentions that he had made a solemn promise never to interfere in the Enterprise crew's destiny ... but then he did.
One might imagine that after much contemplation, he decides that since the timeline has already been altered beyond repair, he might as well fulfill his duty to Romulus; the lives of billions of people are at stake, after all.
(In fact, according to the prequel comic, the subspace hypernova poses a threat to a large portion of the galaxy, so we're really talking about
trillions of lives.)
We might presume that Hobus is a threat in both universes; that the potential to go subspace-hypernova was sealed into its formation, millions of years ago.
Furthermore, you could throw in some treknobabble about how it's a macroscopic (macrocosmic, even) quantum subspace phenomenon, and as such, there's no guarantee that it will explode at the same time it did in the Prime universe. It might never go off ... or it could off tomorrow.
In any case, so long as Hobus remains, the Romulans will never be safe.
(...and again, the Countdown comic suggests that neither is the Federation.)
The olive branch
So, imagine Spock Prime decides to provide the Romulans with information about the Hobus hypernova. And he does so through the Starfleet Diplomatic Corps, so that it can be seen as a gesture of good will from the entire Federation.
This sets the stage for a joint science mission between the Federation and the Romulan Star Empire; the birth of a peace process, and perhaps a step towards Vulcan-Romulan reunification.
== Plot synopsis: Prologue ==
I'd like to see the Enterprise transporting a delegation of Vulcan scientists and diplomats to Romulus, including one particularly important VIP: Spock Prime.
During the trip, there is ample opportunity for both Kirk and Spock to speak to Spock Prime.
He still refuses to tell them about their possible futures, but instead focuses on their shared traumatic experiences with Nero.
Kirk and Spock are both professionals, and on an intellectual level they know better than to hold the entire Romulan species responsible for the acts of a madman with too much power.
Nevertheless, through their conversations, Spock Prime notices that they still harbor some resentment against the Romulans on an emotional level. It's important to note that both have grown up with stories that depict the Romulans as bogeymen, and their main contact with Romulans prior to Nero's incursion was reading about the Romulan War in history class.
So, though they realize Nero was not representative of the Romulan people, the incident nevertheless served to reinforce a prejudiced view that they already bore with them.
Spock tries to alleviate this by telling them of the Romulan people's many virtues: Their philosophy, their culture, their art ... and, most important of all, their diversity.
He explains that while some Romulans still resent the Vulcans who they feel banished them from their homeworld, most desire nothing more than to live in peace.
If one wishes to throw in some cameos, the trip between New Vulcan and Romulus could provide some opportunity for that.
We migh imagine Kirk having daydreams about Kirk Prime, or about Picard and Geordi putting the finishing touches on the Jellyfish.
When he informs Spock Prime of this, the old Vulcan is noticeably troubled. He informs him that his father suffered from
Bendii syndrome, a Vulcan neurological illness which can cause a loss of emotional control, sometimes manifesting through the projection of emotions onto others.
He mentions that the illness is suspected to be hereditary, so he has long wondered if he would fall prey to it. However, since the other Vulcans in the delegations have not yet picked up on it, the illness can't have progressed very far; the fact that Kirk has had these visions must be a result of their mind meld on Delta Vega.
== Plot: Conflict ==
Once on Romulus, the Vulcan delegation goes with a small security detail to confer with their Romulan counterparts. Representatives of the Enterprise crew, meanwhile, are to be led to their lodgings before a formal banquet with Romulan dignitaries.
On the way there, Kirk falls to his knees, screaming with pain. Two seconds later, a loud boom is heard.
It's not long before they discover what's happened. The Vulcan delegation (together with their Romulan counterparts) have been targeted in a terrorist attack. What Kirk felt was a vision of Spock's overwhelming final sensations, transmitted through the telepathic connection they developed in the mind meld on Delta Vega.
The Romulan government condemns the attack, and the loss of life on both sides. They promise the Federation their full cooperation in the investigation, and that they will dedicate significant funds to the endeavor.
Now we ask ourselves...
Can the Enterprise crew keep the science mission and the peace process alive in the face of such a tragedy?
Can they trust the good will of the Romulans, even when their worst preconceived notions have just been so spectacularly confirmed?
...and even if they trust the Romulan government and the majority population, can they ever truly have peace with a culture that houses such destructive elements?
Fringe elements they may be, but they are nevertheless capable of so much destruction.
Killing Spock – Rationale 
I realize that killing Spock Prime reeks of shock value, but I feel that there is something beautiful about killing off the series' most iconic and long-standing character on the eve of the 50th anniversary.
Even more so if he dies in the line of duty to the ideal of Vulcan-Romulan reunification, which has been an important aspect of his character for 25 years.
Last but not least, it would cut the umbilical cord between the two universes, once and for all, and put the new crew to an incredible test.
Give me some time to figure something out in my head (something which will be in accordance with my 'guidelines' above: no old enemies, no threat to Earth, no insane officers-gone-bad and no knock-offs of Khan/TWOK)... I'll get back to you with something soon. 
