Point for:
Nexon do look like a logical choice to take over the running of CoH (they run Maple Story, they're pioneers of the F2P system, etc etc...), and their involvement would, of course, also nicely explain why NCSoft don't seem interested in offers from outside for the CoH IP - as Nexon and NCSoft are buddies now, NCSoft could just cold shoulder any offer then *ping!* just effectively hand over the IP to Nexon and claim Nexon had "made the offer that promised the most for the future direction of both companies" or some other such corporate waffle.
Point against:
Timing. When you close an MMO, there's always a legal wind down thing - the 'sunsetting' period. Fine, if you're closing the game for good, but not clever if your intention is to close it just so it can be immediately reopened again. This is because as the 'sunset' time is ticking away, NCSoft is losing players to other games, and alienating the ones who remain by their continued silence and fluff statements. Why close the game at all? Cryptic's Star Trek Online used to be run by Atari, but they hoofed the operation over to Perfect World to run instead, and the game never skipped a beat.
Closing with a view to developing CoH2 makes even less sense - look at Guild Wars.
When CoH was shut down, Guild Wars was bringing in less money for NCSoft than CoH was...but there was no way they'd close GW when GW2 was due out - that would tick off the GW community pretty badly. The fact that they didn't give a stuff about offending the CoH community speaks volumes about their regard both for the IP and the playerbase I think.
Could the CoH IP be sold to Nexon so they could develop CoH2?
Not likely - remember, of the two companies, it's NCSoft that is the more successful developer, so why get the weaker of the two partners to develop a sequel? Better for NCSoft to keep the IP, develop CoH2, then have Nexon market and run it...but if that was the plan, it would be more logical to maintain player interest by keeping the original going until such a sequel was released. Just like Guild Wars.
No, basically I think that while the question of Nexon running CoH is a very sensible and valid one, the answer is moot because the only logical time to get Nexon in to run CoH was while the game was still running. If you wait til after the game is closed (or even after you've announced that the game is closing), you've already lost far more than you could claw back by keeping CoH alive under new management.