As for Rewrite anime, I think the best way to do it:
1st season(2 cour): 8-10 episodes of common route + rest of episodes for
Kotori/Chihaya mix with bits of Shizuru and Lucia.
Akane will be difficult to mix with other routes so I'd say just animate it as separate AU OVA. (like Tomoyo/Kyou routes in Clannad anime)
2nd season(1 cour): Moon + Terra
Expand Terra with greater roles for Chihaya,Shizuru and Lucia+more romance with Kagari.( This is just wishful thinking though)