With all due respects the prospect of either happening are slim to none and (as the old saying goes) Slim just left town.
Yep. Think about what would have to happen for Cosmos to join MLS.
1) MLS becomes a promotion/relegation league with NASL as its second league, and Cosmos win promotion.
God knows how far along this is, if ever. A league that is still concerned about salary caps and that is earning a lot of keeps by getting money for franchises isn't exactly looking to screw the owners (including the new ones of NYCFC) by diminishing the value of the franchise by allowing relegation.
2) MLS decides New York needs a third soccer team and chooses the Cosmos.
Right now there is only one league with three teams in NY/NJ area: the NHL (Rangers, Islanders, and Devils). Everyone else has two (Jets/Giants NFL; Yanks/Mets MLB; Knicks/Nets NBA). For this to happen not only does NYC need to show it can support three MLS teams, it has to show that a third team in an existing MLS market is better for the league than a new market in say San Antonio or St. Louis.
Ironically, the only way that possibility even becomes conceivable is it the NYCFC and RBNY franchises start doing really well. Although I suppose there is the possibility that:
3) Either NYCFC or RBNY fail, and Cosmos then lobby to become the NY2.
This would likely require New York to prove it can't handle 2 MLS markets forcing one of the NY clubs to fold or relocate.
4) MCFC, realizing the MLS has become a threat to the EPL, drops NYCFC or the Red Bull heir sells off the franchise for the sake of buying a zoo of kangaroos or whatever it is Australian heirs buy and the Cosmos rise to take the place
Moving on.
5) the Red Bulls realize that in the wake of NYCFC that being a club for the sole purpose of a marketing venture isn't a great idea and therefore buy out the Cosmos logo/franchise so that they have an actual crest and name apart from the brand on their shirt rather than reverting to the Metrostars.
That this is maybe the best shot they have bodes well for how unrealistic this is. Cosmos had the chance and passed.