IMO ketamine is pretty much supplementary, I consider it pretty much standard with MDMA (and most empathogens/psychs). Kind of like a bread and butter type of thing and not really a flip. Flipping drugs is slightly different than just combining two nice drugs. I dunno, maybe that's an abundance type thing, and I guess I'm talking semantics regarding flipping; they definitely work well together.
2c-b and MDMA isn't quite as good as I expected - I don't think they have a particular special synergy as per LSD And MDMA. Don't get me wrong, it is good - my best experiences have been at night time at a psy trance stage not really being 100% what was going on but with the music being all crisp and relentless and merging in with everything.
Mushrooms and MDMA - I can't really remember that well to be honest, but my dose of mushrooms was low and the MDMA was taken pretty early to compensate (after an hour or two). I've taken a dose of mushrooms that presented the perfect opportunity for MDMA, but I didn't have any.
IME the best type of flips with MDMA involve taking a psychedelic and forgetting who you are. When you start to remember and have some semblance of things... this is quite naturally going to be just as the peak is coming off... then you take the MDMA and it brings your ego back with a force... and in style. It creates a very humble, hedonistic, utopian and unique state of being/mindset. You have to lose yourself to really find yourself. This should probably work well with most of the safe tryptamines/lysergamides; 4-aco-dmt, 4-ho-dipt and the likes. Anything too stimulating and/or empathogenic will not work and could be dangerous i.e 5-meo-mipt, amt, etc.