A good idea for the substitutes would be decreasing Linda's and Amid's HP growth by 20% and their Speed growth by 30% and giving them after that change minor Holsety Blood. It always annoys me seeing Leaf/Janne getting even more OP Weapons (SO MANY GOD DAMN A RANKS) while the real Magic user are just getting some ,,El-Wind/Fire/Thunder tomes". With minor Holsety Blood each of them will be able to have a A Rank tome (While Janne gets Bolganon ). And from Story related things it would also make sense. While Ethnia marries some unknown guy in the normal FE4 she would now marry a unknown noble from Silessia in your Patch :0
The reason behind this, is boosting some substitutes that kinda deserves better Weapons consindering they don't have pursuit or 7-8 Move.