I would make sure that a fully modded, max rank warframe is only twice as powerful as the non-modded version. Same for weapons. And I would make sure that the best weapon is at most twice as powerful as the worst weapon. And this would be the rule going forward for any new item being introduced.
Rationale: reduce the power creep, and allow player skills to play a bigger role (as opposed to right now where the right loadout just trivializes any challenge with brain-dead gameplay). This would likely make the gameplay more engaging. And this would make the whole arsenal relevant: play with what you like, at worst it's only half as good as the best gun.
Also, each forma would add a very tiny bonus to the Warframe or weapon, like +1% HP or -1% recoil.
Rationale: reward the time and dedication of players who like to grind, without making it too OP that it trivializes the content. So if I have a main frame that I play all the time, I have a forma sink and I get something in return.
Overall idea: make sure the fashionframe is aligned with the gameplay. True endgame.