Lunatic Spellslinger

This subclass takes a wonderful spellcaster and turns it into something twisted and hilariously useless. It encourages the removal of all utility in exchange for a large damage number. Think of Megumin from KonoSuba.

Arcane Overcharge

Using this feature, you push spells past their usual limits. Similar to a Paladin's Divine Smite, you expend spell slot(s) to increase the damage on a single spell attack. You can expend a number of spell slots equal to half of your level, with no limit on the level of slots you can expend. After using this feature, you become incapable of moving or even doing anything that requires any physical effort. You return to normal after a long rest.

Extended Incantations

You extend the casting time of a spell in order to increase the effectiveness of the spell. For each round you hold a spell (counting as concentration), the spell's damage increases by a number of damage dice equal to how long you hold it. The spell also gains the Siege attribute if you hold it for a number of rounds equal to the spell's level.

Whilst holding a spell in this manner, you cannot use your action, bonus action, movement, or reaction until you release the spell by ending concentration.

Improved Destruction Magic

Your Overcharge and Extended Incantations gain increased effect.

Overcharge - No longer restricted to require an attack roll on the spell

Extended Incantation - Spells gain the Siege attribute after a number of rounds equal to half of the spell's level. The spell's damage increases by a factor of how many rounds you spend holding it (i.e. if you hold a spell for 3 rounds, you multiply the spell's damage by 3). Spells gain AOE depending on how long you hold it (held for 1 round, a melee spell gains AOE of 5ft, 2 rounds = 10ft, etc), this part of the feature increases a spell's initial AOE by that same amount.

Ultimate Destruction Magic

Your previous features become even more deadly, to the ultimate degree.

Oversurge - There is no restriction on how many spell slots you can use with this ability. When you use this feature, you use the maximum possible damage for any rolls made.

Extended Incantations - Your spell's gain the Siege attribute after the 1st round of holding the spell. The damage increase of this spell becomes exponential, for example holding a fireball for 3 rounds will exponentially increase the damage by a factor of 3 ([8d6]^3). In addition, for each round you hold the spell, the spell's area increases by 10ft.