The fortigate is running on Router Mode (NAT).
I have few ASA's connected behind the Fortigate, and I have assigned routable public IP's to their legs.
I have pointed the routes towards the Fortigate. (ie, For ASA's the next hope is Fortigate for the internet destinations)
And in Fortigate I have given policies like
Source -- ASA Leg IP( Public IP ) --- Policy for Initiator
Interface -- IN
Destination - Remote Peer IP
Interface -- OUT
Service -- UDP-500, UDP-4500
NAT - Disabled
A Reverse policy is required if the ASA wants to receive the IKE packets from the Remote peer and act as an Responder.