the fact that people find this to be a fun feature makes me think it won't be tournament legal.
no offense, but pros seem to be very... anti-fun.
atleast, the ones i've come across certainly are.
I'm sorry, but I just have to quickly reply to this. I think that if you really think about it, you can see how this statement makes no sense at all. Competitive players play a video game... Because it's fun. It's really not that complicated.
I'm guessing you're referring to some of the competitive communities' harsh reactions to random or unbalanced elements in the game that you might consider fun. However just because you don't understand the reason why, doesn't mean it isn't a good reason.
I'm a competitive player and I would love to see stage morphing be implemented in some tournaments. It's hard to predict the full consequences that morphing brings to the table though, so I guess we'll wait and see.
Some more stage combos that seem fun to me:
Great bay -> Pirate Ship. Setting sail
Pirate ship -> Rainbow cruise. Bonus points if the transition occurs during the tornado section.
Spirit train -> Pilotwings. Trains planes and... (Big blue?) Automobiles?