I think that the last evolution of Simon should he becoming some kind of hippie or radical ecologist, or both. Like he loves the plants, nature in general, he would not want to use violence to solve something since he has already saw a lot of it in his life and he meet a god I think that would give him a new perspectives.
Simon trying new clothes with more colors than black and greys and oppaque tones because he found out that he likes more vivid colors and patterns, and maybe loose material.
Simon interrumpting Rocky and Grace experiments in the biodome, because he wants to know what kind of effet could it have in the ecosystem of the biodome.
Rocky realizing that Adrian and Simon would be besties, because Adrian is also an ecologist.
Grace explaining to Simon what the climate change is and how it is affecting Earth, and how they had to destroy antarctica to gain more time. Simon, who has never see this planet with his own eyes, crying like the climatologist.
Simon making sure that Grace and he recycle everything they have in the house, because he refuses to just expend resources mindlessly.
Simon having his own garden or forest (I always say this, but he always deserves it).
Simon practicing yoga and meditations in his garden or forest to try to calm his thoughts.
Simon wearing a lot of necklaces and bracelets with themes of nature and the stars.
Now I also have ideas for the Modern AUs, because I can:
Simon would not use a motorcycle (I know is cool, but there are not ecologically-friendly) but a bicycle just like Grace, they could have a cute-meet because of it (think in the possibilities).
Simon been a well now activist against companies that damage the environment.
Simon going to prison now and then because he was stop in a strike.
Simon who can still have his background of having been part of a cult and been a convict for having help to kill people with a bomb, coming full force to fight climate change because perhaps in this way I can make a change for the better.
Grace, at least in the book, has also strong feeling about the climate change and those who doesn't believe in them, so he could have meet Simon in one of this strikes or in some event related to the environment.
Grace rambling very passionately about all the damage that has been done to the oceans, and how it could be corrected if people really tried, and Simon realizing that he is in love with him.
Simon arguing with someone who doesn't believe in climate change, or making a speech about it, and Grace realizing he loves him.