Spellsmith

Spellsmiths can create new spells to pass unforeseen obstacles at the drop of a hat (or rather a Short Rest) and combine their spell slots to cast more high-level spells than any other class! But there is a limit to how many custom spells they can have in existence at once, and spell slot fusion is markedly inefficient, especially at lower levels when few spell slots are available to combine.

Level 3: Spell Crafter

You gain the ability to spend a Short Rest focused on your Spellbook, creating a new spell at the end of it. You gain no other benefits from the Short Rest if you do so. If you attempt to create more spells than your Proficiency Bonus, you must choose one existing spell to replace. They must be approved by your DM for you to use them. You can also create edited duplicates, e.g. making Fireball deal Force damage and have an Intelligence save. You can also do this as part of a Long Rest at no cost.

Level 3: Slot Fusion

You can take the Magic action to combine two spell slots of equal level, creating a single spell slot that is one level higher. The spell slots created in this way cannot exceed the level of your highest available spell slot (regardless of whether you have already used that slot) and vanish when you finish a Long Rest.

Level 6: Raw Power

When you cast a damaging spell, you can change it's damage type to Force. If you do, any creatures targeted by it automatically fail saves and it automatically hits on any attack rolls, plus it deals the maximum possible damage. However, you take Force damage in return equal to half the damage dealt by the spell. This Force damage ignores Resistance and Immunity to Force damage for all targets including you, but you can only use this once per Long Rest.

Level 10: Improved Fusion

Once per Long Rest, when you use Spell Fusion, you can increase the level of the resultant spell by your Intelligence modifier instead of by 1. It still cannot exceed your maximum spell slot level.

Level 14: Master Spellsmith

You can create a new spell as part of any rest, and you add your intelligence modifier to the maximum number of custom spells you can have in existence. You can also make minor tweaks to existing spells without a rest, at your DM's discretion - for instance, you could make Ray of Frost deal half damage in exchange for a 20ft speed reduction, but not make Meteor Swarm have half as many damage dice per meteor but twice as many meteors.