I would also like to know what to do w/ thrawn.
1) in arena, I'm not sure your team is the best set up, but I would guess get rid of tfp (too squishy) w/out auto taunt to start or GK. Zader you need for lead, DN b/c he is meta, savage for the cleanse and utility on defense.
2) I guess you need potency mods and avoid speed secondaries so fracture stays longer?
3) I'm not sure how to use him in rancor, if he will be good w/ zader, ep, tfp, jyn etc.
4) for HAAT, he can replace RG in your chirpatine, sthan, sun fac (and then GAT) for p3.
anybody else have ideas for 1-3? my advice is based on what I've read and seen, not personal experience yet. I have 7* him last night but not really sure what to do w/ him for rancor or arena or mods