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..."
This is how I would love to see Spike someday...standing before an insurmountable enemy, against odd that everyone knows he can't overcome, and not budging. Only way this could be better is if he were dressed as Kick-Ass.
Spike's true self is good. He has greedy instincts but unlike other dragons he's capable of fighting them and winning. He had to work hard to overcome those instincts and learn to be generous. Spike's biggest faults are laziness and jealousy. That's possibly why he's so attracted to Rarity. Not only is she a knockout but while he had to learn generosity it comes naturally for her. It probably impresses him that her generosity is basically a symptom of her clearly severe obsessive compulsive disorder. Her generosity seems to be a compulsion to overcompensate for a usefulness to the group that she probably obsessively worries about lacking. She didn't have to make them those gala dresses pro bono and she should have put her hoof down immediately over most of their inane suggestions but no she had to be giving enough to risk both going broke and getting ridiculed. Thus Spike stays attracted to her mostly because her generous aura rubs off on him and helps him keep his instincts suppressed and at bay. Having an inspiration to remain good gives Spike more power to fight his instincts and proves that deep down he is good just like deep down Discord is neutral.
Actually, I wouldn't say laziness is one of Spike's faults, since he works hard as Twilight's assistant and everyone's little helper. When he actually displays a slothful side its most likely due to exhaustion from the constant tasks he's done every single day. The hardest workers are usually the heaviest resters.
As for jealousy that more stems from from his biggest fault, insecurity. His greatest fear is being rejected so when he's ignored for somebody like Owlowicious for long periods that triggers his insecurity and leads to intense feelings of jealousy.
That's very true about his dragon hoarding instincts since those are just natural born instincts for dragons, like Squirrels feel the need to gather and hoard or humans to socialize. But he's never displayed them until Secret of my Excess as Twilight says its unusual he's so grabby there. That is a good way of describing his attraction to Rarity beyond a simple crush, especially after his dragon instincts were triggered and after Rarity's encounter with Discord since she knows what its like to lose control to avarice. I would really like an episode were Spike's insecurities about his dragon side defining him are explored and where he realizes those don't define his real self. They did that in Dragon Quest which was a good nurture over nature augment but something more like this picture would really go a long way.