1. Change out the trigger to an SOA style trigger, remove the disconnector from the trigger bar.
2. Upgrade/change out the hammer, springs and sights.
3. Polish all of the internal contact surfaces, extended safety, extended mag release, frame and slide refinish, new barrel bushing, Nill grips.
Not everyone is going to benefit for all of theses modifications. But you said most possible upgrades so i threw in as much stuff as i could think of.