Remember the days of building infernos and xbows After war was declared and then cancelling them, rinse repeat next war. And SC put an end to that.
Well... how about... the enemy you match with is the enemy you face on war day? No more... I'll add a weapon after match. No more "Oh looky my TH just upgraded from TH8 to 9 so I'll add all these extra weapons and a queen" or "Oh sorry fellas, my TH9 hit TH10 and just too bad you now face infernos and extra xbow and stuff."
If the CC that starts prep day cannot be altered on the base, how about making the entire base not able to have New stuff from war match.