- Modularity - a design the grows with the users needs
- Standards compliant - you'd want to support mounting for at least ATX, EATX, and Mini-ATX
- Ability to add a PCIe backplane - going back to modularity (might not be necessary - but if modular, could be a separate part!)
- Physical support for graphics cards - a lot of longer cards can warp when stacked vertically, this can cause problems for fans getting caught on the next card's solder
- Open case if you're targeting a price point, this will help keep it low. If price is no object, then a closed case with modular cooling to again grow with the user.
Another point to consider is the viability of GPU mining to make an investment worth while for users. BFL will be making an announcement this Friday regarding their BitForce SC (ASIC). Again, if it were me, I would maybe hear what they have to say before investing a lot of time or money into this. (Unlikely example: BFL may announce something that renders GPUs unprofitable by a large margin, and that they have enough supply to meet demand - which could lend you a niche to market some type of enclosure for the ASICs?)
As long as carpentry and machining are a passion of yours, I'm sure you will have fun with it whatever you do. If you do fun first and profit later, you will always win!
Subbed to see what comes of this!