Within the Judgment Day film (and the film's novelisation) there are no indications who the T-1000 was based on. He's simply described as having...
...a handsome face, with strong features framed by military-short
brown hair. His gray-blue eyes were furrowed with deep concentration,
confident. - Randall Frakes - Terminator : Judgement Day
and
A NAKED MAN glides from a shadowed doorway behind the cop. Nothing special about him. Certainly not built like a terminator. The flash
of light and fact that he is naked are pretty good clues that he just
arrived from the future. His features are handsome bordering on
severe. His eyes are gray ice. Penetrating. Intelligent.-
Terminator : Judgement Day Screenplay
The director of the fourth Terminator film (McG) spoke about the idea of incorporating an older Robert Patrick into his next film as a scientist who is working on nano-technology:
TheArnoldFans: Will we possibly have a return of Robert Patrick in a CG form?
McG: Funny, I talk to Robert Patrick with great regularity. It's interesting and i'll bounce it off of you guys (McG begins to lower
his voice and leans in closer to me so the other press tables don't
hear). I mean, I like the idea and the perspective for the next
picture that you meet Robert Patrick the way he looks today...and he's
a scientist that's working on, you know, improving cell replication so
we can stay healthier and we can cure diabetes and do all these things
that sound like good ideas...and to once again live as idealized
expressions as ourselves.
So imagine seeing a 60-year-old Robert Patrick and KNOWING 'holy shit' that's GOING TO BE the T-1000 who comes back perfect, lean and
the whole thing (McG makes the T-1000 "naughty naughty" finger swaying
movement). I haven't concluded that but Robert and I had dinner the
other night. - TheArnoldFans ComicCon Interview