The only obstacle is Alex Kingston's schedule, which is why we had Tasha Lem created for Time of the Doctor.
Is there actually any proof or record of that, or just assumption? Because it conflicts with what was written in the big magazine special about all the 50th anniv stuff.
That says - based on interviews with Moffat I believe - that Tasha Lem was supposed to be from a race that are all psycho killers, that grow up all constantly trying to kill each other, and that Tasha had worked hard to set that aside and become Mother Superious. That she had nothing to do with River.
I personally think that the theory that Tasha was meant to have been River in the script - or actually
was River in another incarnation or something - is purely down to Moffat's repetitiveness as a writer and inability to create women of different types. (For example, that description I wrote of Tasha up there also sounds exactly like Vastra as well.)
River isn't my favorite character....but if Captain Jack is there too, I just might have my mood changed.
And again here - to my mind, River and Jack are basically the same character. Both raised in the 51st century, both fast with a gun and a witty quip, both sexually fluid and available, both fell in love with the Doctor, both have a timeline that loops over and around itself in confusing ways, both have tragic childhoods and secrets that even the Doctor doesn't know...
I know Jack was created by Davies, but he was introduced in a Moffat script, and I sometimes feel like Moffat just wanted his own version of the character but made her female so nobody could copyright him.
when he decided she was the daughter of Amy he made her ridiculously convoluted.
Moffat 'decided' River was Amy's daughter as soon as he started working on season 5, or else Amy's name would not have been Pond.
.