One of the things that came up in one of the multitudinous discussions RE: Thrawn in the past few months that I really resonated with is that he is a tactician. So many of the suggestions for Thrawn are just arbitrarily great and require little to no tactical skill to use. For instance, reducing all command dials by 1 is a super strong effect, but all it does it make everything require even less strategy, which then is not in keeping with the theme of the character.
Taking Thrawn as your commander represents having access to his tactical genius. An ability that's just "arbitrarily good" is the perfect representation of Thrawn's tactical skill.
Reduced dials would represent his ability to change tactics on the fly to accommodate a changing battlefield, as well as his ability to predict his enemies responses.
This could work as long as it says something like " can not be used in conjunction with Relentless" because an ISD with only 1 command dial would be ridiculous especially if coupled with a Navigation Officer, Engineering Captain, Wing Commander or Tactical Expert
Also would it apply to all ships in your fleet? That would have to cost at least 40 odd points