I know that to get optimal space when bridging, you should bring your heels as close to your hips as possible. Thankfully I have long calves (better angle on my bridge) but it could be better if I got my feet further back.
My problem is knee flexibility. I can get my heels to my hips when aided by weight (like sitting seiza) and maintain it for extended periods of time without pain, but without it, I've got about 8" of space open. I've started to notice this causing problems in other areas like getting hooks in for certain sweeps.
Someone pointed out to me that you shouldn't stretch joints, and this is something I've wondered about since I picked up Stretching Scientifically a few years ago. I guess I'm left with the question of whether my knee issue is caused by "flexibility" of the actual joint, or if ligaments around my knee are what the problem is. Since sitting in seiza helped me before, I may try doing that again.