I think they held onto them specifically to prevent MG/Rover from using those names. Triumph really would've been the perfect badge for the 75 and 45, and a Riley version could've been made eventually.
Given that Triumph and BMW's products used to compete head to head in the seventies and that Triumph's sporting image would be direct competition for BMW, I don't think we'll ever see a new Triumph.
A Riley version of the new Mini might be a possibility though - a two-door sedan that's longer and posher rather than the tiny little hatchback, just like the original Riley Elf, which was a big mini with a trunk.