I'm running a campaign involving a certain amount of mass ground combat and naval combat. I'm mostly using the Unearthed Arcana mass combat rules, because it lets me keep a feel for the grand combat happening in the background and zoom in on the players during the high tension bits. But RAW spells have one big disadvantage on this scale - lack of range. Fireballs and Lightning Bolts are regularly outranged by longbows (Also, I houserule that Lightning Bolt has a 200ft range because how often are you actually firing at more than two or three targets in a line that you couldn't just hit with a fireball for the same damage from further away? 100ft vs fireball's 150ft really makes fireball the 'always' choice over lightning bolt.) and in naval combat they are ridiculously outranged by cannon.

The only spells that actually compare are at 9th level, things like meteor (1 mile) and storm of vengeance (sight). But since I don't want 20th level wizards running around willy-nilly in my campaign, that's not practical.

One thing I have seen a lot in fantasy novels is the idea of wizards casting artillery spells, specialised for range or which require teams of wizards to cast because of their power requirement. Here is my starting feat with that in mind. Please break this horribly so I can see what I need to do to fix it.

Artillery Spellcaster
Prerequisite: The ability to cast at least one spell.

You have learned to extend the range of your spells beyond their usual limits and how to co-operate with other spellcasters to do the same.

- When you cast a spell (excluding cantrips), you may cast it using a higher level spell slot to increase its range. At 1 spell slot higher, range is doubled. At 2 spell slots higher, range is quadrupled. At 3 spell slots higher, range is octupled, and so on in a geometric fashion. Using a higher spell slot in this fashion does not increase its damage, number of targets or other effects, even if the spell description ordinarily states that casting the spell at a higher level does so.

- It is possible to mix expended spell slots to increase range and effect. e.g. Fireball, a 3rd level spell, does 8d6 fire damage in a 20ft radius sphere at a range of 150ft. Using a 4th level spell slot to cast it with this feat would allow you to increase its damage to 9d6 or to do damage in a 20ft radius sphere at a range of 300ft, but not both. Spending a 5th level spell slot (one slot for improved effect, one for range) would allow you to create a 20ft radius sphere that does 9d6 fire damage at 300ft range.

- You can combine spells with other spellcasters with this feat to extend the spell's range. All spellcasters must cast the same spell, producing only a single instance of the spell regardless of the number of casters involved. Each spellcaster adds the maximum range of the version of the spell they cast to the maximum range of the final spell. i.e. Three wizards cast Fireball, two at 3rd level and one at 4th. They create a single fireball, which will do 8d6 fire damage in a 20ft radius sphere at a maximum range of 600ft (150ft + 150ft + 300ft from the 4th level spell slot).

- When combining spells to extend range, if a spell can be cast at a higher level to improve its effects, the final spell is equal to the power of the weakest version of the spell contributed. i.e. Three wizards cast fireball, each using a 4th level spell slot. Two wizards cast a standard fireball (8d6 fire damage) but using a higher slot to improve range to 300ft each. The third casts an improved fireball (9d6 fire damage) at the normal range of 150ft. The final fireball only deals 8d6 fire damage, at a maximum range of 750ft (300ft + 300ft + 150ft), essentially wasting the third wizard's extra spell slot. If all three wizards instead cast an improved fireball at normal range (9d6 fire, 150ft range) using a 4th level spell slot, the final fireball would do 9d6 fire damage at 450ft range.