"who's going to take over that [heel] spot"
Maria.
Obvioulsy the notion is laughable and it goes without saying that I'd rather Natalia, but Maria's pressumably in for some kind of rub from Ziggler (And let's not forget, WWE have seen fit to give her at least one title shot in the past!).
Easy booking really... Get the belt back on Melina and she can fued with Maria, Neidhart & Layla until McCool comes gunning for her again.
I don't expect to see Natalyia receiving anything of a push in the near furture - They haven't even currently got room in the booking for a dominant 'Glamazon'.