I think we don’t have to use spoiler for the whole text anymore, seems like everyone already opened it :D
I still don’t think Kouta or Micchy would turn into Overlords, as I said already, I think they are Overlords as of now. I mean Kouta is, Micchy will be one when he gets his new lockseed.
Why would they transform into Overlords Inves? The Overlords where created when the species from the old world genetically enhanced themselves, that wont happen with the Riders. As stated before, I remember Rosyuo and Takatora talking and Rosyuo said that both races choose their path, the Overlords altered their DNA and the humans used the belts. Kouta with Kiwami arms is the human equivalent to an Overlord Inves.
Further mutation might happen, but not into an Overlord Inves, that wouldn’t make sense.
The only thing I can see happening is that Kaito chooses to become an Overlord to escape his sickness.
Concluding this, I have a new, clear pic of Micchy's new lockseed!
It reads "L.S.-Yomi", Yomi being the Japanese word for the land of the dead, the world of darkness. Spooky!
Micchy, what happened to you, you where such a nice kid, and now you've become the prince of darkness!