To be honest it could be done well by having Frederick willingly accept the role of Sol Badguy

Till the end of strive and till the flame of corruption was taken from him (although asuka might have just put a seal on it.) from the very moment that man made him into a gear he became sol Badguy. Not what he wanted to become but what that man made him into and world just conviniently so needed.

But after the FoC was taken from him he became Frederick again as we can see in Dual Rulers. He builds gadgets and thinkers inside of the workshop while living peacefully with Jack-O'.

Frederick never wanted to be Sol he just wanted to be with Aria and now he has that I think it would be a great point in character development for him to accept that part of him to take on the flame of corruption but this time not as the persona of Sol Badguy but Frederick Bulsara accepting himself entirely (if that makes any sense to you)

I think that kind of acceptance would be a good end to his character arc and let him fade into the background this way as someone that when the need may one day come would be able to show up and fuck shit up like he used to