If you want more people splashing Resolve, you could consider giving it objective control strength.

You nerf overall damage against Structures and Epic Monsters by 5% and then give the Resolve Tree the added bonus of +6% damage to Epic Monsters and Structures. You might want to nerf one of its other two stats a little, though, in compensation.

Overall this makes Towers tankier because not everyone is going to run Resolve and Tanks have low damage against Towers anyways (aside from Demolish if they take it), and it reduces damage in the game because more people are going to be splashing Resolve.

You could do this with other Trees, too, to make them more attractive to people other than their main users. Want more people taking Precision other than Marksmen? Give it ... uhm ... % base Health Regen? Fighters have high base Health Regen and like the sustain in lane, right?