While I know its almost impossible for it to actually happen in the show, I've always wished for an episode where Spike is forced to finally face the greatest fears inside his heart, such as being rejected by Twilight, his friends, and ponies in general due to him being a dragon and a potential threat to everyone if he grows out of control again, which might lead him to hurt somepony.
Then, as his fears come together and take the shape of his greed-grown form, causing him to start giving up and succumbing to them, the Mane6 find a way to talk to him through some spell and manage to cheer him on by telling him how important he is to them and how they've always considered him one of them. With his confidence renewed thanks to his friends, and a new warmth in his heart, Spike confornts his fears which causes the giant Spike to shink down into a copy of his normal form, which then fuses with him, effectively taming his dark nature.
Spike returns to the real world with his friends, feeling lighter than ever, and since that moment onwards, gains the ability to tap into his inner greed (now directed towrds his friends instead of a hoard) allowing him to temporarily grow whenever its needed.
"Why won't they recognize me? Pay attention to me?! LOVE ME-" "Enough with the hot air, idiot!" "Twilight! Rarity! Recognize-" "I said shut up already! Yeah, I often feel starved for attention and recognition for what I do. I want them to truly acknowledge me as an equal and not just as some office hardware piece. You're not some mind-shattering, life changing revelation! I'm not greedy because I'm a dragon, I feel greed because I want something, someone, anything or anyone to be there for me! Because I want to fit in! Because I am you and you are me..."