Or keep the ET’s free/open, but have a bunch of challenges and rewards tied to certain ways.
For example:
Completing a set with only sniper kills would be a challenge, and completing it would reward you an “Arcade skin” Sniper.
Complete a set with only fiber wire kills would be a challenge, and completing it would reward you an “Arcade skin” Fiber wire.
And so on.
That way the ETs are any/any and are free to approach as you want, and the challenges and rewards could motivate players to come back and use different approaches.