Crushing Field has an 8 second recharge, which is exactly the same as the AoE immobilize in every other control set. If you're referring to Gravity Distortion Field (the AoE hold), the 240 second recharge is exactly the same as the AoE hold in every other control set.
The lack of -kb in Gravity's immobilizes is a defining feature and shouldn't be changed. If you want AoE damage, you can pick that up in your epic pool. Very few of the control sets have significant AoE damage. Gravity received a major overhaul in I22. I think it's largely OK as is.