It's very easy to do.
Raise the requirements of all armors. Right now, the highest requirement for an armor is 16, which allows you even to do an agi build (18/21) at lvl 30 that can wear the maximum armor in game, having 7 ath, allowing you to be efficiently fast.
Raise the requirements so the milanese is 27 requirement, and scale down all of the other armors requirement in range to this. This way, at lvl 30 you have to make a 27/12 build at best to wear maximum armor, making you slow as hell (as well as not having much wm). Ath build with 18 str should be able to wear medium/heavy armors (heavy kuyak/sarranid guard at best, if not lower).
Same for weapons, the requirement for maximum damage weapons should be around 24/27 str and not 18 as it is right now (flamberge definately 27, 24 for poleaxes, glaive, danish, german, etc) , thus forcing you to go the str way to have a high damage weapon, so you become a high damage output character, but slow.
The changes should be in the way for you to choose between being fast or strong. Right now, you can be effectively both things (wear highest armor without making a str build ... which is senseless) with a balanced build.
I always thought that is senseless in this game that you can use top items just having half of the top reachable amount of an attribute that serves as requirement. Is the only time i saw this in a game and it's really weird. It should be about 70-80% of that, not half.