I've started to work with Rhye when civ5 was released, but he was very busy IRL at the time, and I also had my own mods to develop.
I made the Historical Spawning Date mod and Revolutions mod for civ5, based on my own code in lua, I don't see anything (except free time to develop it of course) that could prevent a RFC for civ5, especially now that the source code is out.
For civ6, until the source code is released, I can see some limitations, related to the AI (especially the -current- inability to control AI units), and control over diplomacy (still, it may be possible to neutralize this one and make your own) but Spawn Dates and Revolutions, no problem, even without the source code.
I don't plan to make RFC for civ6, I have my own vision for a Civilization total conversion mod, and I wont pretend to be able to do something as good as his mod, but give me time, and I'll eventually release something on that scale.
You can compare civ6 and civ4 mods in the context of this thread, I think it's unfair considering the time since release between the two, still it's a fact that civ4 modding scene is bigger, but I don't think that you can use that comparaison to determine what can and can't be done with civ6 related to mods, knowing that the source code will eventually be released.