Anything is possible? Well maybe not quite...
Certainly...from what we hear the WinPho8 platform is not yet proving to be the saviour that Nokia might have hoped (even if the M$ pocketmoney is helping to keep the Finns afloat).
Currently Nokia still have a fair position in the "budget" end of the market - basic phones for people with basic needs - particularly useful in the developing world where battery life is probably more imporant than being able to download 245726 fart-apps. However after losing their way with Symbian on their featurehones and smartphones, they decide to jump ship to MS, and are now finding that ship isn't in much better shape. It certainly isn't proving to be a gamechanger...at least not yet.
So what are the choices?
1) Apple iOS? - Not a chance - hell will have to freeze over before Apple will even allow that notion to engage a single braincell for more than a nonosecond!
2) Android - the 800lb gorilla....an obvious possibility, but as others have commented, the challenge will be how they can successfully differentiate themselves from the hordes of droids out there.
3) Maemo/Meego/Jolla, etc - not beyond the realms of possibility, but I'm not sure how well it would fare in a shootout with the now more mature Android, and you still have the problem of building marketshare and app ecosystem.
4) Something completely new/proprietary? - not impossible, but would require a LOT of investment. Do Nokia really have the resources to do that? I doubt it. Also has the same issues regarding marketshare and app ecosystem as 3).
5) Resurrect Symbian? - well as Elop said...anything is possible....but this one might well be running a close 2nd to option 1) in likelihood! Again would require a LOT of investment to bring back up to scratch and regain marketshare/app ecosystem.
So, are those the choices we have? Well there might still be one more option:
6) Blackberry? - A bit left field you might say, and RIM have themselves shown signs of struggling in recent years, but there maybe some logic to this suggestion. Both Nokia and Blackberry have historically been strong in the business sector, and in recent years BB has grown popular with the younger market, due in part to BBM. RIM also have a half-decent app store with a good range of apps. With the exception of Nokia Maps, Nokia's store has become a bit of a relic. Nokia has good hardware pedigree and RIMs market share, at least within the business and youth market, means they must be getting at least somethings right on the design, UI and software front.
So, could a partnership with work in practice? Good question! They have been strong competitors in the past, but they are no longer the "big fish" that they were (although they are not minnows either!). There would certainly have to be a meeting of minds between these two companies to make this work. However neither one of them is dominant now, so perhaps there is better grounds for cooperation to fight their common enemies (iOS and Android). And for Nokia, a dalliance with BB might be less offensive to Microsoft than one with Android.
So, on reflection, Nokia and RIM jumping into bed might make some sense, and if it works out, then who knows, maybe we will even hear weddings bells!
Anything is possible! ;-)