PuRplE
1 No, the power of Sunshine just runs counter to him being a Spirit of Winter, maybe a Hie Hie no Mi (ice-ice fruit) with no weaknesses to boost that aspect of himself. 2 No, adding Erza just seems odd, him and Madoka have a good thing going so far and it’s still a bit early for romance. There may be more fitting girls for him in the future.
Rather then giving him Sunshine Magic... I'd prefer you expand upon his race; maybe give him some traits of the winter fae from Dresden files? Sunshine Magic is kinda opposite of his nature... Winter being caused by the earths rotation; bringing further distance from the sun- Giving him sun magic on that level seems off. Not to mention it being demonic magic. If we stick to the theme of winter & him embodying said concept... Then upgrading his race into a winter fae. Especially as just joined *Fairy* Tale- It would be kinda fitting as a reward for this mission. As a fae he'd get a much needed firepower boost for marvel... Maeve/Mab are around solar system level; attack potency if I recall and have an intuitive understanding of magic- I suggest you look it up... It would really give him more potential.