for size, they should be fitted, but not as tight as pipe gloves. somewhere in between. Maybe 1 cm of thickness around the fingers and palm.
Material: Leather palms and fingers/tips. Goretex top.
Cuff: Should be tight around the wrist with elastic stretchy that is tight enough to keep moisture out, but not too tight so that it restricts being able to get your hands in. Also it should have an elastic strap that Velcros closed. Use the most durable velcro possible.
The liner should be micro-fleece. Make it tight inside so that it doesn't get twisted and bunch up. Liners that bunch up really suck.
It should also have a secondary cuff that is made out of fairly tight elastic, and this should dangle about 10 cm from the glove, and be attached by some webbing or adjustable string. This way you can put on the secondary cuff first thing, and then just let your gloves dangle while carrying gear etc. This way you will never lose them. This strap should be removable via a very secure metal clip in case the customer is not a fan.
THe ipod/iphone finger tip is a good idea. Maybe if you had a micro-zipper that was water-tight super sealed, but that was designed so that it was still easy to undo and re-do with you other gloved hand or your teeth. But this is a risky addition. It's also easy to just take off the glove when you need to control your gadgets, especially if you're wearing the secondary cuff-- outlined above-- that prevents you (by letting it dangle) from having to hold the glove under your armpit or in your lap (if on the chair) and risk dropping it.
All seams should obviously be super sealed, but also tripple stitched. You could also consider putting a reiniforced bit of leather or even hard plastic (like downhill longboard racers rock) under the palm material for those of us who ride rope tows during the early season jib park days.
colour should be brights mixed with neutrals. Maybe white leather palms, black gortex on top, and then a bright forest/herb green between the fingers.
vents are useless if you opt for a highly breathable goretex for the top/backhand of the glove.
you could also consider inclduing a tight, form fitted marina wool liner for extra cold days. The word wool is usually assocatied with scratchy, 1800s type stuff, but Marina wool is extremely comfortable, doesn't smell, isn't synthetic, and wicks moisture better than synthetics. It's also pricey, but a small form-fitted liner shouldn't increase costs too much. Make it as tight as possible (like surgical tight) without sacrificing any comfort.
those are my ideas. Stoked on seeing your final product. peace