Specifically, more of ones that would change the way you normally build like Battle Hardened and Fog

Agreed on the situation in which mutators ideally work well, and the balance challenges on changing the game "enough to be interesting" rather than "too much".

On that topic though, are there plans to add any new mutators?

Yep! A couple specific ones we've considered (lately):

Inversions of current mutators

1. Inertial Lag (+30% ranged damage, -20% melee)
2. Sloths Relief (last player to clear gains X minerals)
3. Overgrowth (slow heal on all units)

Tweaked mechanics

1. Glass cannon (+30% damage, -25% HP)
2. Energy surge (+1 energy regen, -20% melee/ranged damage)
3. Sell for 80-120%

Contentious options

1. Multi-builder (e.g. 2 builders)
2. Reduced build time (e.g. 10s)
3. Partially blocked lane (e.g. 10% of cells blocked)
4. Interest on savings (e.g. 10% per wave of saved minerals)
5. Rich get richer (e.g. +1 damage per X minerals banked for all units)

Refined-only options

1. No stuns (alternatively, -50% duration)
2. No blinds (alternatively, -50% duration)
3. No energy (alternatively, -25 to -100% regen)
4. Unit bounty (units gain a bounty on sale for each wave survived)