The biggest issue I have with my kneepads is that they slide down. Any fasteners, such as velcro, etc. need to be very strong. In some cases it could be the difference between a nasty bruise and a cracked kneecap. If pads don’t stay on the place that they need to protect, they’re useless. I only wear mine at the bike park, because they suck to pedal in, so stuff that’s well-ventilated and don’t bunch up and chafe at the back of the knee would be nice. Fitting farther up the thigh helps them stay on. If they’ve got a hard shell at the front then some sort of hinge that allows the knee to flex freely would be nice. You might be able to get some ideas from medieval plate armor; as far as hard shell designs go, they had to keep the wearer alive and able to move easily.
As for helmets, the less I can notice it, the better. Good ventilation, padding, and well-positioned straps help.