Hextech should benefit your team instead of denying your opponent.
In it's current state, TFT has way too many options to deny your enemy.
With hextech being introduced it's starting to get over the the top. Games are starting to become oppressive and unfun.
I honestly think hextech should be a 'buffer' to your team.
Something like...
Hextech 2/4 - Increases item effects on allies by 15% for 8 seconds
Hextech 4/4 - Increases item effects on allies by 30% for 8 seconds
Having 4/4 hextech with frozen heart for example would be...
'Adjacent enemies Attack Speed is 55% Slower' for 8 seconds. (After the 6 seconds it would go back down to being 25%.)
Obviously not every item can benefit from this BUT I think it's much better than hextech in it's current state.
TLDR
Make hextech an item buffer for your team. There is too much denying your enemy in the game and it's starting to get unfun.