I guess I have problems understanding why Mattel wouldn't want to own the rights (entirely) to what are essentially their characters. Someone, like say Dree-Elle, is a MOTU character that Mattel cannot make...which seems a little bizarre when you think about it.
So I assume the catalog of Filmation characters is prohibitively expensive - otherwise, why not just buy the whole thing back and control your own property?
All that said, I will continue to hold out hope that we at least get a few Filmation characters in the line - although most of the characters I'd most want would be POP ones (Shadow Weaver, Madame Razz, Scorpia and Sea Hawk in particular.) Certainly wouldn't turn down Lizard Man, Icer, Shokoti, etc. though.