I did say Natsu is END. However we don't know how END is going to play a part in this. We only go by how he's going to be able to form, so in the end, it's all speculations until we see how END is resurrected. IT should be interesting to say the least.
There's also theories around that Gray has MM in his disposal since he might have absorbed MM when he fought Marde Geer, and it may be his trump card to deal with both END or Zeref. Again it should be interesting to say the least.
I think relevance really does matter in this manga, because it's mostly relevance that caters for characters getting to shine. Jellal may have more build up but he doesn't have more reason. Jellal didn't have his life ruined by Zeref, he had his life ruined by Ultear thanks to her brainwashing. Gray had his life ruined directly by Zeref through his creations. Big difference. Zeref played a huge key role in how Gray's past was traumatic, filled with misery, and anguish. He was one of the top three characters who was negatively effected by Zeref and hence has a huge reason to fight Zeref. I feel Jellal mostly wants to get rid of Zeref for a noble quest which is why he's been taking down dark guilds after dark guilds and the reason he formed CS, so in his mind he can be redeemed of his sins (the ones he's really not responsible for, as it was Ultear's fault). His beef with END is mainly part because it's his father's mission and last request to him. Once he finds out END's true identity, he will try his best to save Natsu out of it as Gray is one of the most smartest characters in the guild.. So in the end, most of his anger will be directed at Zeref, because in some way he may be also linked to how END was formed. Mavis will be the one to give the final blow , but Gray and Zeref are the front runners to fight him as well.
I wouldn't call it basic either, just low-mid tier spell in his DeS mode. Something like Ice Geyser, or Ice lancer would be basic.
Nice name btw