Right now in terms of functionality, Grenade kit and Mortar kit fulfill incredibly similar roles as ranged attack kits. Grenade has four main perks over Mortar, 3x hits for traits, better damage coefficients on cooldown abilities, 1 dedicated Bleed ability, and Grenade Barrage. Mortar kit has higher range, applies fields, is worse at applying conditions with the exception of Flash Shell which blows Flash Grenade completely out of the water, has a healing ability, and a blast finisher toolbelt. It's like this because Mortar wasn't designed as a kit in the first place, but now that it is, it could be time to change either it or Grenade kit.
Let's say that Grenade kit is replaced by a Bazooka kit that fills the ranged niche. Mortar kit cooldown abilities are buffed to Grenade kit levels and Bazooka gets Rocket Barrage instead of Grenade Barrage. Engineer now has one kit that fires arcing projectiles and one kit that fires straight projectiles. Would anyone feel a particular loss for Grenade kit in general?
Let's flip that around. We have a Bazooka or Portable Cannon kit as an elite kit. How important are the fields/projectile finishers from Mortar kit? Or is it alright that Mortar is so similar to Grenade kit?