My bathroom is in a dormer. I keep my house cool (55F) most of
the winter, and heat the room I'm in with small electric heaters.
The bathroom has two wall mounted lights, with two prong
unpolarized outlets. I was running a small (600W) heater off of
the light, until I discovered that the lights run off the old
knob and tube wiring in the attic, which are partially covered in
loose insulation. (!) This is the original lighting circuit
(15A)that feeds most of the overhead fixtures installed when the
house was built (c.1920).

I want new electric service, new panel, and removal of ALL the
K&T and old BX wiring in the house. Then I can INSULATE (yay!).

HOWEVER, that will not happen this year. I need to run a circuit
up to the bathroom from the basement to at least power a heater
(600-1500W). I'd like to not have to re-do it when I re-do the
rest of the wiring.

Bathroom will need, eventually: lighting circuit, an exhaust
fan, a heater, and a courtesy outlet (shaver, hair dryer etc.).

Code requires separate circuit for a bathroom heater? Only if
it's fixed? Plus, a GFCI for the courtesy outlet by the sink?

I'm wondering what the most practical solution is-one 12-2 for
the heater for now, or a 12-3 to power the heater and the
courtesy outlet? I have a GFCI receptacle and a 20A GFCI breaker.
I haven't bought the wire, because I can't decide which would be
best.