I dunno, I wouldn't mind seeing some sort of combination of cluster and mortar. The new mortar could have similar range to the existing mortar, but it would have a detonation effect similar to the cluster. If the mortar projectile is not detonated at all, it's AoE would be similar to a grenade, but the damage would be slightly higher than a grenade. If detonated in mid-air, the projectile would be similar to a detonated cluster, perhaps with more shards and/or damage (and more green). If detonated on the ground, it would have a similar 'boom' to the current mortar. You would be unable to lay the projectile down as a mine, ala current cluster, there would be a amount of 'arming time' where the projectile needs to fly through the air and you would only have a second or two to properly detonate the projectile after it hits the ground before it explodes into a regular, non-detonated mortar explosion (perhaps the projectile could 'blink' a darker green than normal to signify that it's about to explode as a 'boom').
This weapon would still have usefulness and power, but would take more skill to properly utilize. Spamming would be diminished, as the mortar user would need to time their detonation to make the weapon effective. If they tried to spam cluster shots at the enemy base in midair, it would be fairly useless as the clusters would detonate too early and be more of a pretty firework show than an effective napalm-style strike. Standard mortar 'boom' spamming would be reduced quite a bit as the player would need to wait for their shot to land then properly detonate it (so no more fire and forget 'booms').
The reason I like this weapon is that it still promotes proper skill and timing, and if utilized properly, can still be effective but removes that spammy nature of the original. It's cluster functionality makes it useful against fast targets (with proper timing) while the boom mode would make it also useful against slow and/or stationary targets (base turrets, sleeping HoF).