Guilliman has come back and the rumors are that he will be creating the new breed of Space Marine. Here are a few tidbits that have floated in.
Please remember that these are still rumored bits.
via krazynadechukr on Dakka
My local Gw manager (who told me about a new thunder hawk, overlords, and death watch before public knowledge) said the new marines are RGs new breed. Old marines are still valid and these will have new stats. They're between custodes and current marines in the line up of things....
via Sources on Faeit 212
Sorry, the new marines will not get their box sets for awhile, "near end of summer" and there will be new assault marines and terminators coming along shortly thereafter.