I do like the concept of using concentration is a form of "upcast". Cast X, get Y. Cast X (and use concentration) get Y + Z.Also, I fixed Daylight. If you decide to concentrate on the spell, the light produced is sunlight.
I think its opens up a lot of good design space for abilities, especially at high levels when your concentration has high competition so you can come up with some pretty solid upcasts and keep it balanced.