You can learn a lot from what other companies already did, good and bad.
Try that. Then you will have to figure out where you want to be placed on the unstable triangle between materials (durability), weight and features. Unstable because if you want good durability and good features, the weight will suffer. One of them has to suffer, you can't have all three.
On features Briggs Riley and Tumi are both very good. I'd give the notch to BR for luggage and to Tumi for briefcases, tied between Tumi and BR for duffles.
On durability BR is already very good. Red OXX is excellent but not great for features and not great for looks. Andiamo Valoroso was the best for materials and also very good on features and quite nice for looks. But they didn't have enough marketing and then went south.
Tom Bihn is a much smarter brand in that regard. Durability is good but no unconditional lifetime warranty, yet some of the features are clever and the looks are pleasing at least to some though I wouldn't call them chic.
Rimowa has some excellent products and excellent service and marketing. However, the wheels on the spinners are indeed fragile (if the best rolling) and the corners of the poly-whatever cases are also weak points. Prices are premium level indeed and I feel at that price point a lifetime warranty is needed. But given that they are German the thought doesn't cross their mind.
I realize you didn't ask for that at all but maybe it helps anyway.
Oh, and if you make a dopp kit, make one that weighs no more than 5 ounces and can be washed or rinsed. Dopp kits are total weight killers for the light traveller.
Duffles and briefcases must have exterior pockets of different sizes. There must be something to quickly slide in a magazine or newspaper. Something for the odd gadget like headphones or sunglasses. And a way to attach them to the extension handles of rolling luggage.
On a duffle the handles are very important. Comfortable carry handles and a good shoulder strap are a must. Look at the Optech straps and their cousins from Tom Bihn. Grab handles on the ends are also an excellent idea. Check out images of the Valoroso VD-25 if you want to see a perfect pocket layout and perfect features, that no one else got right after them.
Wallets should be flat so you don't look like you are wearing a gun when you carry them in your breast pocket. A travel wallet should have a slot for a small and slim pen, always needed to fill out documents or jot down a quick note. RF shielding would be a nice plus if it can be kept thin.
Good luck!
Till