In the interests of simplicity, cutting the thickness in half and adding some trim along the edges would probably be sufficient. Caution tape might work but may be too noisy, maybe just one of the i-beam textures would be fine.
Adding lumps just gives more things to bonk your head on underneath the platform, which may be important because there's a health kit down there so it's going to see a lot of traffic.
Switching from the displacements to the fence is a huge improvement though, it makes the whole area feel more open without actually changing the play space. Plus it gives you more terrain to go nuts with detailing. :O