My niche is building scale airplanes and this doesn't necessarily have to mean large scale either. I figured this out by trying a little bit of everything and only progressing from one type of plane until I got good at the kind I was working on.

This progression order sorta went like this with the airplanes I can remember:
U-control-(shoestring & ringmaster)
Gliders-(gentle lady, sophisticated lady)
Gliders with powerplants (Gently Lady w/.049, Electra)
Trainers (Eagle 63, Kadet, PT-40)
Low Wing Trainer (Tiger, Sportster 40)
Stand Off Scale (Piper Cub)
Bi-Plane - (Sportster Bipe, Smith Miniplane, Skybolt)
Sport (Somethin' Extra, Uproar, RAV-4, Agwagon ARF)
Speed (quickee 500s)
Combat (Macchi)
Aerobatic -(Midwest Extra 300, Kyosho Breitling ARF, Ohio R/C Extra 300)
Float (Laker, Cub on Floats)
Scale (1/4 Cub--BUSA & Sig, 1/3 Cub, Stafford Commanche, Waco, P-47)
Scratch Build (Somethin' Extra 150%, Stafford Commanche)

Well today I can say scale is my thing but after doing the list I guess the best answer is that the niche will change as you go along. One thing did seem to stay constant, I tended to stay away from ARF's and electric.

Right now I'm finishing a P-38 that I've been working on about a year now. I think for the next plane I see what that whole park flyer craze is all about.