I think the Mass Driver is as close as we need to an infantry-based mortar weapon.

Anything else will be fine to happen via artillery turrets.

Not saying it's a terrible idea, it might be fun if CCP do it anyway. I just don't think it'll be needed.

[CURRENT STATE OF THE MASS DRIVER]
Right now the mass driver basically functions as just a grenade launcher that mainly works in close range because targeting long ranges can be very difficult, but I think that is a waste since the grenades have the potential to reach much greater distances than they are typically used in. There are great possible applications for this weapon that can greatly improve its usefulness to the team, and make it more versatile to allow different playstyles.


[PROPOSED NEW "MORTAR" FUNCTIONS]

Aiming down sight should be removed, and replaced by some sort of targeting system for mortar-style artillery fire for long range support.

Targeting option 1: That targeting system can be an augmented reality launch arc that shows an estimate of where the mass driver grenades will land; firing the mass driver should be impossible when using the targeting arc terminating at close to mid range to prevent abuse in close to mid range.

Targeting option 2: A large 3D hologram projects out from the player's arm screen (the one that shows up when hacking), or as augmented reality 3D map taking up half of the dropsuit's vision (and making the user vulnerable as a tradeoff). This should show the currently spotted red dots while the player holds the mass driver with the other hand, and the hologram would show approximately where the mass driver grenades will land if fired with a marker.
Option is more favorable since obstructions could block the line of sight to enemies when using the arc.

While in the "artillery mode", the mass driver should fire all rounds in the magazine at once as a scattering cluster near the spot highlighted by the targeting system. This would help cover a wide area of effect (useful for groups, and moving targets), but would have the drawback of having to reload with every shot.

[IMPLEMENTATION]
Option 1: All mass drivers could get this secondary function.
Option 2: A new "artillery" variant of the assault rifle could be added for these long range functions. Such a variant could have low splash damage, but have a higher splash radius to make up for innacuracy, and target movement.


[WHY THIS WOULD BE GOOD]

Makes the mass driver useful beyond just being a "noob tube" by offering a new support role.

Great for supporting allies.

More long range combat variety.

It would depend on teamwork since only spotted enemies would appear in the targeting system, so the user will have to rely on teammates to spot enemies to target.

[WHY ITS NOT OVERPOWERED]

It would only be useful for dealing with long range large groups as support fire since grenades travel slow, and targets tend to be mobile making it unlikely to get kills.

The targeting would be imperfect, so a chance of landing hits on someone consistently would be low (especially direct hits).

TL;DR: Its not much, just read.

@KAGEHOSHI: I like your MD variant.
I should mention that artillery rounds, especially variable-speed, can be made to impact the target area at the same time with the proper firerate and angle adjustments. Maybe you could have the targeting system interlink with the dropsuit servos to make that possible?
It would still be a "scattered" cluster with the same total damage, the main difference is that it hits as a single strike - so the first round doesn't give the enemy enough warning to escape!
Of course, the shooter is stuck in one place while the gun CPU is controlling the suit's aim...

When rigging comes out, an augmented reality sight that traces the firing arc and marks the blast area - EVEN when it's going to land behind cover would be an awesome addition - that way you could use the blast in conjunction with a friendly spotter to line up shots on enemies behind cover and at longer ranges than normal.