Been thinking a bit more about the Hydra/Fire Bolt/Meteor changes, and I think following would be the more complete changes that are desirable:
Hydra: Change Damage Synergy to Hydra of Fire Bolt to 6%, remove the synergy from Fire Ball. (Damage stays the same, but only takes 60 points for build, allowing for greater build flexibility).
Fire Bolt: Add Knockback effect to Fire Bolt. Change +16% Fire Damage synergy from Meteor to +16% Damage Synergy from Hydra. Add new +20 Physical Damage synergy from Meteor (not a percentage, flat damage, so maxing out at 400 physical damage on a Fire Bolt with 20 points in Meteor).
Meteor: Cut the time to impact in half. Convert 50% of Fire Damage to Physical Damage.
With those changes, against Monsters vulnerable to Fire you’d cast Meteor, and then spam Fire Ball while waiting for the casting delay to run out (or for trash mobs just cast Fire Ball without bothering with Meteor, it’s that good), but for Physical Immunes you’d cast Meteor and then spam Fire Bolt while waiting for the casting delay to run out.
Meanwhile, for a Hydra build instead of casting Fire Ball after deploying all your Hydras, you’d cast Fire Bolt, which while doing less damage than Fire Ball has the effect of Knockback which actually works quite well with the Hydra playstyle. It would also work well with Thunder Storm, making that a viable hybrid with Hydra in addition to the more typical Frozen Orb.
You could also do a 100 point build with 20 Fire Bolt, 20 Fire Ball, 20 Meteor, 20 Hydra, and 20 Fire Mastery for the highest fire damage possible with Physical Damage on the Meteor/Fire Bolt as your back up for Fire Immunes.