I signed up for the forums to say just a few things:
First, Angilasman is absolutely right and DC should have reverted BIlly to the Captain Thunder name. It's not quite as cool as Captain Marvel, but it ties in well with the magic lightning and it's a name Marvel Comics has never used.
Second, Mary has as strong a claim to the Captain Marvel name as Carol does, given that she used it during the Ordway Power of Shazam run.
Third, the one true Captain Marvel is, was, and always shall be the android with the detachable limbs who fought the one, true Plastic Man (among a few others) in 1966 in a very short-lived series from M.F. Enterprises.