Giving her fighting role or giving her a character building role
The thing is, I don't see how to give her a character-building role without her being involved in the Grail War stuff somewhat. They would naturally tend to try to keep her as far away as possible for the duration of the war, for her own safety. I would also like her to have at least
some fighting role, even if it is only at the very end.
Plus, that in itself implies she's not one of the main characters, because she is mainly there to build the characters of the
actual main characters. At least, that's how it comes across to me....
At very least, I don't think the battle where Ilya is kidnapped should be the final battle. Mainly because it would either be too easy for them to win (by Rider backstabbing Gil at the opportune moment), or else Sakura's involvement would be very limited. I think that it would be the penultimate battle, with Shirou, Rin etc. about to lose horribly when Rider suddenly turns on Gil, allowing them to rescue Ilya, but they haven't yet managed to work out a plan for actually
defeating Gil. Then, they return home and plan out the final battle, with Sakura's assistance and with her (and Rider) playing at least some role in it.
I also think Sakura should have been training to use her magic. Even though Shirou and Rin don't want to involve her in the war, they would still want her to be able to defend herself if she gets dragged in
anyway. So, they'd encourage her to learn and would assist her in doing so.