As the community is probably painfully aware of by now, I'm a bit obsessive about well-defined class roles. I've already talked about having Limpets and AT Mines swap classes with Support getting the more fitting Bangalore Torpedo as a Limpet replacement, and now I've got something else in mind.
While I do like the more mobile nature of the Grenade Crossbow over the Mortar, it just isn't fitting in the Support class, so I have yet another move-and-replace suggestion. The Grenade Crossbow would be moved to Assault (where it's much more fitting) while Support gets the Leach Trench Catapult.
While it might not seem all that different in function, the Leach Catapult's mobility would be more akin to that of the Rocket Gun, requiring the user to rest it on a surface in order to fire it. This makes it more defensive and thus more fitting for Support while still being more mobile and simpler to aim than the Mortar.