Dynamic NAT

Current scenario: Static NAT (SNAT List) where I have the IP of my LAN tied to a Valid IP working 100%, today I have around 10 and as a business rule these IPs cannot be shared.

Future scenario: They will expand from 10 to approximately 400, that is, in the current scenario I will need to register 1 in 1, my concern is not to register, but maintenance and management of this.

Given the scenario presented I need something dynamic, where I need to continue doing NAT 1: 1 and as long as the valid IP has been used, that it is not shared, it can be reused by any LAN IP when the existing connection goes down, then it is released the table.

Someone already needs to do something in this sense or has something, I imagine they need an iRule (a little complex) to meet this.

Thank you

Adriano