Realistically, F1 is (or should be) a non-starter because there are certain criteria that need to be met in order to apply for, let alone, grant the super licence required to compete. Rossi meets none (although money talks, so no doubt strings could be pulled if someone wanted it enough and got dizzy seeing the £££ signs - eh Mr Ecclestone...)
Secondly, he has no experience, no form, so who would want to put him in a front running car when there are plenty of young drivers with proven form and plenty of experience from the lower formulae, as well as the current line up which is fairly rich in talent itself. Maybe there could be an option in one of the tail end cars if he brought enough money with him but that would be purely to do just that, money and exposure for the team which could generate more money. Buying a ride in other words, just to get on the grid with no hope of winning.
Finally, even in a Red Bull or McLaren, he'd get nowhere near the current drivers. Forget the Ferrari/Marlborough PR stunts, that was never 'testing for Ferrari', it was having a play. Sebatian Loeb, no slouch on tarmac, properly tested a GP2 car a couple of years ago with a view to switching and although very quick, admitted he wasn't quick enough to compete with them.
Rallying in a private team is obviously an option as he would have the backers and money, but a front running works team, no chance. Competing in those mickey-mouse gymkhana events like Monza is worlds away from the real WRC, as shown in his outings in the Rally GB. There are no credentials for a works drive.
WSB would be a good crowd draw and money spinner and promoters/sponsors dream, but would he want it? Not sure, he has a big ego and maybe only Motogp success can satisfy it. It's the pinnacle of tarmac racing so surely anything else would be a step backwards and an admission that he was no longer good enough. When you're as good as he is and a proven winner, competing just for the enjoyment of competing, often isn't enough, the enjoyment is in the success. If success is no longer achievable, then retirement usually follows.
Motogp. Never rule out completely the option of a private Yanaha or Honda team, but I wouldn't put money on it personally. A works ride with either of those two seems most unlikely. There are enough younger riders to fill those seats now. Ducati is probably still his best option of getting success, if they can overcome whatever the difficulties are with the bike - issues which seem very clouded and unclear at the moment, with all the double talk, contradiction and confusion of opinion that seems to pour forth with every press release from rider or team (Hayden to be fair has been fairly consistent and enthusiastic about the bike) Success though, can only be in the form of odd wins or podiums, title success is surely no longer realistic, although no doubt you have to believe to keep going.
If, come the end of the season, the partnership still isn't working, I can't see it continuing into 2013, even though he has stated he wants another 2 years. I think it will be retirement, because even on a private bike, he is always going to have the doubt in his head that the bike isn't on a par with the factory team and if results didn't come in the first few races, I think the belief that he could win again, along with the will and enthusiasm, would disappear quickly and for good.