I am working on my own bag, and like the directions you are going, I am definitely interested to see what you come up with.
For other projects I have done, sign shops actually will often give away scraps large enough for any piece you need.
I have thought about vertical storage on the sides (as mentioned here) but it makes any rain fly design more cumborsome to get in and out.
The water bottles on the front as pictured here visually is cool, but I am worried its moving more weight away from your body, which will make the bag feel heavier.
My hydration choice will likely be a blatter of some type, and then have a nylon strap/pocket for my thermos.
I have some ideas for your stash pocket. In previous bags I have put a D ring in the valuables pocket to clip my keys to (so they don't get mixed up with other stuff) and a velvet pocket stitched in for my phone so keys and other items are not scratching the screen, and it also keeps it easily accesses able.
Don't forget little things like a convenient mini pocket as well as pencil holders and a safe place for the score card. (The lack of mini pocket on my very early Gorilla boy bag bothers me a bit)