1. Fire-Shell of leIG18 isn't worth it, replace it with something more useful.
-> AT-mode for leIG18. Used like US ammunition-change on tanks. It changes the leIG18 into a small PaK, new weapon-profile. Heat ammunition, low angle shot, fast shell speed. DPS like Panzerschreck.
AT mode for leIG 18 would make it far too powerful. 270MP, already effective against buildings, now able to fight off light vehicles. Why on earth would you ever build a Raketenwerfer?
Miragefla implemented a Hollow Charge single-shot ability for the leIG 18 previously. It wasn't too well received; not enough damage by itself, yet giving it any form of stun would make it OP.
In my opinion, simply make the flame barrage more effective (increase performance). I would increase shots fired per incendiary barrage and/or increase AoE and DoT.