Holy crap, are you a mind reader? I had this story idea eating away in my head for awhile, even with a melee droid that replaces G0-T0. While I find G0-T0's story and motivations interesting, he is essentially a useless companion.
Basic Background:
I was thinking after seeing the results the highly successful HK-47, Revan (or Malak) would commission another droid to specifically combat Jedi even in melee combat. It would be based on the HK line but could include components and characteristics of Krath war droids and/or the Mark VII experimental assassin droid (the droid Revan encountered on Korriban). It would go on to kill many Jedi during the Jedi Civil War where it would be acquired by G0-T0 to accompany the Exile in it's steed.
I don't know if you can create a new droid model, but since it's based off of the HK line, a re-skin of Hk-47 should be sufficient.
KOTOR II Role:
Since the droid would have components of the Mark VII experimental droid, it could also grow to appreciate life and regret its actions during the Jedi Civil War. It would be another light sided droid, but unlike T3, it could be swayed towards the dark side (let's say through justifying killing to help the many or something of that nature). For example, it could be appalled by your actions if you force persuade those muggers on Nar Shaddaa to jump but the influence could be gained if the deaths were justified in helping that refuge. Perhaps it could also have an existentialist crisis.
His intro dialogue could be reused G0-T0 dialogue but with this droid in its steed and more dialogue to cement this new droids place. When HK-47 confronts G0-T0, this droid could be by his side, and have dialogue to his former boss. Maybe he could be a companion to HK during his assault on the factory, but then again the two would probably not get along that closely. His interactions, with the other companions could be a replacement for G0-T0's interactions. It would be amusing to see Hk-47 call him "the fat one". Since Bao-Dur's remote never gets any love, this droid and remote could have friendly conversations with each other on their places in the universe and other droid stuff (like motivators, upgrades, and boys). This droid would also be able to fill in G0-T0's role in the side quest to determine that Goto is a droid.
To make him unlike G0-T0, the droid actually has to be useful enough for mission consideration but not too overpowered that he always used no matter what. Since he's an assassination droid, his skills shouldn't be high like T3's. If he could use lightsabers (or other melees), he would essentially be another Bao-Dur without the tech specialty or force powers. I guess to balance that out, he would have the droid upgrades and attachments to use in combat. Though another idea would be a unique attachment that acts just like Mira's wrist mounted rocket launcher (that could work on either HK models). Take his abilities to use ranged weapons (say his programming gives insufficient skills to use one) so it would not be an exact HK-47 clone.
This is just what's going on off the top of the head right now. If you like what you read, I can provide more details and ideas. No matter what you go with, I hope you go all the way with this mod. There's already too many jedi companions and all the other companion replacements just add even more jedi.
