I think rail seating would be perfect for most areas of the ground. This allows you to stand up but also rest back against something if a little less steady on your pins. My Dad has had both his knees replaced but sitting actually makes them worse...

The ability to shout or sing is definitely hindered when seated and, as others have noted, half the fun of going to a game is being with your mates. Anything that allowed you to stand together is to be encouraged. Perhaps you just buy a ticket to a part of the ground with rail seats but no designated seat? this would also encourage larger walk-up crowds I'd hope