The Rules are 'Ask Your DM'. And I think the DM should do it because class features you have but don't use aren't making anything more fun.
Well, to what end? "DM, my elf has blue skin, can i have something special?" "DM, my character's daddy is a god, can i have a super power?"
The rules are there so that players don't bombard the DM with endless questions. I do like the idea of a 'special request within reason' for each player and I have done that as a DM, but I'm quite surprised that most of them don't really make use of it.
Back on topic of the OP, i dont think giving the player Agonizing Blast will help him realise his character concept better. Also, the way it should works is, players pitch the idea, and DM come up with the mechanical solution. If you let players suggest mechanical solutions, its almost always in their favour.