Let's first look at the split at the old DPR:
Leica Talk: Every Leica camera welcome
Sigma Camera Talk: Every Sigma camera welcome
Panasonic Compact Camera Talk: Every non-ILC Panasonic camera
Micro Four Thirds Talk: M4/3 cameras from both Panasonic and Olympus
Hence, when the first L-mount Panasonic camera arrived, the old DPR added a Panasonic FF Talk (www.dpreview.com/forums/thread/4323615). That aligned well with the Leica and Sigma forums, but was different from how Panasonic M4/3 users liked to communicate across brands with Olympus users. Hence, the forum was renamed to L-mount Talk.
The end result is that the L-mount Talk forum is the place to discuss L-mount lenses and Panasonic cameras, while most discussions around Leica and Sigma L-mount cameras take place in the respective Leica and Sigma Talk forums. I think that's fine, because a separate Sigma L-mount (or "fp) forum will make exactly zero sense once they launch their Foveon L-mount camera which is more naturally discussed among other Sigma Foveon users.
The hierarchy model of this site is quite nice in grouping together connected forum threads under one umbrella, but some forums clearly belong under multiple top levels, making this tricky. It would be natural to have a Panasonic FF Talk Forum inside a L-mount header, but also inside a Panasonic header.
My suggestion above, based on the assumption that we can't have multiple parents for a given Forum/Category, was to keep L-mount brands together somehow, but not imply that it's only about L-mount. Something like:
L-mount Alliance members
- L-mount Talk
- Leica Talk
- Sigma Camera Talk
- Panasonic FF Talk
That still doesn't solve the Panasonic issue of being split between "Olympus, Panasonic, and Micro Four Thirds" and "L-mount Alliance members", though, so I'm not entirely satisfied with my own suggestion. But adding a Sigma fp forum would not be a solution to anything.