I am currently working on a homebrew Arcane Tradition for Wizard, based around the concept of modifying spells, amplifying or mixing them by directly tampering with the Weave, at the risk of attracting... unwanted attention.
While I am still trying different things for the later features, I would like to make sure that the first feature of the tradition does not break anything. If worst comes to worst, I can rework it and swap it out with another later feature.
Here is the feature as of now :
Rebound Energy: Starting at 2nd level, you have learned to channel the pure magical energy shared by all your spells, and redirect it to augment another spell. When you cast a spell of 1st level or higher, you may choose to expand additional spell slots of any level. When you do so, the spell you cast is considered as being cast one level higher for each additional spell slot level used. For example, when casting a 3rd level spell, expanding an additional 2nd level spell slot, or two 1st level spell slots, allows you to cast your original spell as a 5th level spell. A spell's level can only be increased by up to your proficiency bonus. You cannot use this ability for spells cast through a magic item or scroll.
The intent with this ability is to grant the player the option to give their spells more power, at the cost of using one or multiple additional spell slots. However, I thought giving players the ability to upcast spells at 8th or 9th level early on, even while needing all their resources, could possibly break balance, so I limited the maximum possible augmentation to proficiency bonus (i.e. +2 early).
This limitation is also meant to prevent players from upcasting spells too far past 9th level, if the DM allows it. After all, in this case, a 20th level wizard could use all their spell slots to cast a 89th level spell. This is a lot of resources invested in a single spell, but at that point I think it's fair to assume that there's no way this is balanced ("No, you are not casting a 94d6 Fireball. Yes, even if it costs you all your spell slots.").
In its current state, how does this feature compare with other Arcane Traditions?