While I concur that Mastermind ATO's should be able to be slotted in their primary attack powers, I also think the change should be extended to their secondary as well.
The secondary change would only effect a handful of powers and bring it more inline with how Corruptor secondaries are handled.
This would result in the following secondary powers being able to slot Mastermind ATOs:
Mastermind > Cold Domination > Sleet
Mastermind > Poison > Poison Trap
Mastermind > Sonic Resonance > Liquefy
Mastermind > Storm Summoning > Gale
Mastermind > Storm Summoning > Freezing Rain
Mastermind > Storm Summoning > Lightning Storm
Mastermind > Thermal Radiation > Power of the Phoenix
Mastermind > Traps > Caltrops
Mastermind > Traps > Acid Mortar
Mastermind > Traps > Seeker Drones
Mastermind > Traps > Trip Mine
Mastermind > Traps > Detonator
Mastermind > Trick Arrow > Acid Arrow
Mastermind > Trick Arrow > Oil Slick Arrow
*Please note* "Mastermind > Storm Summoning > Tornado" is the only power from any Mastermind secondary that currently allows for ATO slotting
The secondary change would only effect a handful of powers and bring it more inline with how Corruptor secondaries are handled.
This would result in the following secondary powers being able to slot Mastermind ATOs:
Mastermind > Cold Domination > Sleet
Mastermind > Poison > Poison Trap
Mastermind > Sonic Resonance > Liquefy
Mastermind > Storm Summoning > Gale
Mastermind > Storm Summoning > Freezing Rain
Mastermind > Storm Summoning > Lightning Storm
Mastermind > Thermal Radiation > Power of the Phoenix
Mastermind > Traps > Caltrops
Mastermind > Traps > Acid Mortar
Mastermind > Traps > Seeker Drones
Mastermind > Traps > Trip Mine
Mastermind > Traps > Detonator
Mastermind > Trick Arrow > Acid Arrow
Mastermind > Trick Arrow > Oil Slick Arrow
*Please note* "Mastermind > Storm Summoning > Tornado" is the only power from any Mastermind secondary that currently allows for ATO slotting