Gear Protection

I think you have a few issues with your poll. I do several activities, which the poll didn't allow me to list. Also, what do you mean by design? Functional features? Color choices? I'd buy an ugly pack with good features, but I wouldn't buy a stylish pack with poor features. Likewise, what's waterproof to you? All backpacks of all sizes I've looked at for the last decade or more have all been reasonably rain resistant in their fabric, at least at purchase. They wouldn't be immersion-proof, and extended, heavy rains would eventually soak through via seams and zippers. I might consider getting a roll-closure immersion proof pack, but I don't feel any need for a regularly designed backpack with an extra rain cover. It's fairly common for packs to become less water resistant with age though, so a cover might extend the use for an older pack.

Thanks for taking the time to fill out the poll dabac. I'll definitely update the poll to address the issues you brought up. By design, we mean the aesthetics of the cover itself, so the colours/pattern of the cover itself and how it looks (style).. Functional features would include things such as a reflective strip or other attachments (we're trying to think of features cyclists/athletes would need). We weren't planning on using a very heavy material, because we haven't decided who we should be selling to. If we do choose the athlete market then we would definitely use a heavier material, to make sure that heavy rains don't soak through and so that the zippers/seams of a normal backpack are covered.

To clarify: If I buy a backpack today, I'd expect it to - through the qualities of its fabric - to be sufficiently rain proof NOT to need an extra rain cover.
It's just the most practical approach. It'll cost less in weight/volume to have the rain protection already as a feature of the fabric rather than as a separate item.
For me to add a separate rain cover, there'd have to be something special for it to make sense for me.
Like extending the useful life of an old and worn pack.
Or if the features I want can only be found in packs which aren't rain proof in themselves.

For riding/running/inline skating in traffic/ after dark, I might consider using a cover in Hi-Viz colors and plenty of reflective surfaces.

If it's a team event, I MIGHT be tempted to go for a custom cover to be primarily used as a common team identifier.
It'll probably be more useful than yet another team logo t-shirt. It'd have to be about at the same cost.

But then it'd have to be OUR design. I wouldn't be particularly interested in a TREK, or Specialized or a whatever logo on the back.

For FUNCTIONAL features of the cover:

Those covers I've seen for small/mid-sized packs have relied on a simple drawstring to stay on. This isn't always good enough.
At least there'd have to be some sort of loop/hook to provide a more definite attachment so that the cover CAN'T be ripped off by wind and flutter away into the sunset.
For slightly larger packs a common issue is that the cover isn't snug enough. If facing a headwind they balloon and add drag. Not acceptable in a race/performance situation.
For bigger packs, that don't rest directly against back, it's manageable.
You can basically run laces through the frame/rails to pull the sides together.
This makes getting something out of the pack a bit more difficult, but the cover stays on and doesn't balloon, which are good things.
For smaller packs that do rest directly against the back it gets more difficult. Anything you string across the back panel of the pack might cause chafing.
It'd be interesting to see if you can come up with a design that would manage to deal with this.
Maybe a cover designed a bit like a vest/t-shirt?
Unhook the shoulder straps, pull cover over pack, pull straps through holes in cover.hook straps back up, go.