Howitzer? Something that takes a turn to set up, renders the SPARK immobile but then gives you long range AOE hitting power.

Railgun? Single target, low ammo, high damage, sniper rifle aim profile with high accuracy, for sniping that one pesky unit or doing a lot of damage to a bit target like a gatekeeper or sectopod.

How about bringing in some of the equipment from EW such a restorative mist or mine launchers? It'd be easier to import the assets rather than creating new ones.

Not so much a new weapon, but maybe an ammo-only slot? I really like the idea of these back mounted weapons but might be hesitant to lose the ability to give my SPARKs specialist ammo. SPARK with bluescreen rounds is a great counter to Sectopods and heavy MECs, and has saved my bacon so many times.

@Sturm Krahe
There's already a mod for adding dedicated ammo and grenade slots. However, dedicated SPARK ammo types would be interesting. For example, what about special SPARK bluescreen rounds that not only lower hack defense, but have an additional chance to apply Shutdown for a turn?

As for a howitzer, you're right. We need more indirect-fire options.
I think cannibalizing the scripting for the Gatekeeper could work for deploying/packing. One could create a Pack/Unpack mechanic using the scripting for the GK's Open/Close Shell ability. The only things one would have to change are that said actions would have to cost all of the SPARK's AP for a turn and that they would have to be the unit's first action of the turn, thereby forcing the player to commit to deploying/undeploying, which means having soldiers defend the SPARK while it's changing modes. Oh, and it can't be used indoors because artillery + ceiling = danger-close fireworks.

Also, I LOVE the idea of a proper BattleTech-esque, shoulder-mounted Gauss rifle or railgun. In fact, I think we'd also need a plasma-tier version of it as well: a Particle Projector Cannon (well, at least a beam cannon that makes the vanilla one look like a laser pointer)

Oh, and speaking of BattleTech/MechWarrior, how about this: a two-pack rack of anti-tank missiles.
Advantages:
+ More powerful than a normal rocket launcher (shreds an extra point or two of armor)
+ Javelin-style top-attack fire allows for SPARKs to provide fire support without a clear line of sight, preventing the enemy from easily retaliating; can be used in direct-fire mode as well
+ Smaller AoE, so it's safer to use during ADVENT Retaliation missions

Disadvantages:
- Shorter range than the howitzer or railgun
- No armor piercing
- Smaller AoE than other explosive weapons
- Requires a specific target; cannot be aimed like grenades, rocket launchers, or Heavy Launchers

What about a shoulder-mounted minigun with a AoE attack and\or supression? It can be amazing!