Basically, the idea is to allow doing this:
set interfaces openvpn vtun0 bond-group bond0.
Use case: bonding multiple OpenVPN TAP tunnels over different WANs to aggregate the bandwidth and get faster internet access when MLPPP is not possible.
This is already possible in ZeroShell ("VPN bonding") and most likely in pfSense too (haven't tested it but I can bond ovpncX interfaces in a LAGG).
Under the hood, this would have to be done:
ip link set dev vtun0 master bond0.