I don't think changing memory cards to SD/micro would be the best solution incase current Vita owners would want to upgrade their Vita but will now need a different memory card.
The better solution is to just make the priority Vita memory cards the same price as SD cards. Price really seems to be the main issue for the memory card moreso then the actual type of memory card I believe.
PSP Go was the best system created, so if they can make a new Vita similar is design and features that would have me sold.
That would also be a pretty good solution too. As a current VIta owner I would definitely appreciate the cheaper memory instead of a total switch to SD. Unless of course they happen to slash the price AND switch to SD for the new models which would also be good.
R2 and L2 buttons would also be nice and a better battery life would also be nice additions. A man can dream at least.