About festering spear, leaving a trail won't discourage players to chasing you due the effect of the gas, and mustard on the area will be too similar to the grenade.
But this give a new idea: make the cloud to come from the impaled target. So if a target its hited, it will release gas on nearby hexes (following the main target). This mean that the target can infest their team mates and also it will leave a trail on where he is moving. if the spear mises or its aimed to ground it will only release the cloud like the grenade.
And this makes me think: what about make more spears work this way to act as a different way of crowd control. We can have:
-Dynamite: stuck in the target and explode after time causing AoE damage.
-Festering: stuck in the target for a time releasing a poison cloud
-Energy Spear: Stuck in the target for a time dealing electrical damage and causing AP drain to the target and any nearby player. If energy spear its throw to the ground it will damage nearby players that comes near the impact point and causing AP draining.
I like the idea of a device that disrupt enemy attacks. But perhaps make a trap that do that (point defence drone from SC2?) traps are almost no used, and most support stuff comes from throwing skill so this kind of devices can bring new potential to the trap skill.
And for other throwing weapons apart from spears, we already have the throwing knifes. They can also be looked and reworked to fit a role. Something like silenced shurikens and poisoned knives.