I'm kind of doing the same thing... found a pattern for a nice tunic, but the needle size was waayyy too big for what I wanted. And I wanted it a graciously huge lot bigger.
So I'm looking at the picture and making up my own pattern. Using tons of Fair Isle color patterns iin it, too.
Sleeves? I'm of two minds - whether to knit both [short] sleeves first then cram all those stitches together and knit in the round, raglan style... or do the traditional back-and-forth, bind-off shoulders one. I want to do Fair Isle on the chest area. Can this be done in purl side as well as the knit?
Congrats for venturing on your own. It's really very liberating. Even if you [I] do have to do a couple of serious frogs.