I don't know how hard it is, but I"m sort of doing that right now. I was going to knit a cropped hooded shrug I found on Craftster...then I decided to throw a cable up both sides of the back and a Saxon Braid up the middle...and then I decided to just keep knitting the back until it turned in to full sweater length.
For the fronts I'm just going to do the cable up the sides. I don't want too much cable action going on, especially in the front, b/c it adds bulk to the sweater and I have plenty of my own bulk. Plus I try to play down the boobages and having cables and bobbles across them would probably
draw some attention to them.
It's going well, I didn't mess with things too much as far as gauge and size goes and it doesn't seem to be pulling in too much. I just check the size against a well fitting sweater every now and then to make sure I'm on the right track. Who knows what I'll end up with when I'm done though!
I say knit up a gauge swatch to fool around with in your spare time until Christmas. Throw some cable combinations in there and see how much it pulls, etc. Then you may feel more comfortable throwing caution to the wind when you design as you go. Good luck!