My 2 cents: Rule changes should be shared immediately and agreed upon by the party. The GM is not the rulebook; the GM is the arbitrator for situations the rulebook does not cover. The rules should all be ones the entire group likes, or at least can live with.
Extra content (feats, spells, etc): stick it in the world somewhere and let your players find it. Nothing's cooler than beating a miniboss and finding an entirely unique spell nobody else has.