>>48317120FRLG and HGSS didn't have new forms for old pokemon aside from one-offs like the new Unown or Pichu.
Considering Groudon and Kyogre got new mega-like forms in the gen of mega evolution (and a time where GF is more likely to add on to older mons), it's a reasonable assumption that GF will give something new to Dialga and Palkia.
