Maybe high-light that Willow's magical misuse is a core flaw of her personality from S3 onward; she has an emotional problem and tries to fix it with magic; whether that's unwanted feeling (feeling useless, being in love, being sad) or need for vengeance or carelessly summoning a troll because she got into a heated argument during the friggin' casting.
The addiction-thing always felt like a cop-out because it didn't really address Willow's true character flaw, it just kinda skirted around her arrogance, her emotional immaturity and her need for control. In S6 she took things too far, first with the Resurrection spell (or, specifically, the way she went about it) and then with memory wiping/mind violation/entirety of Wrecked.
Dark Willow is just Willow going full-on vengeance-mode, like had already been established when she went after Glory in 'Tough Love' but more so.
All that S6 needed was to escalate a little from what came before. You could still use Rack and his corrupt misuse of magic, but instead introduce him more organically. An early MotW episode means that the gang have to search him out for info (about a demon summoning, spell misuse, whatever) and that's how Willow meets him at first. She is drawn to how he uses magic (because she's always been intellectually curious) and in a later episode returns on her own and sees how he uses spells and how good/powerful magic user he is. Drops the 'junkie flophouse' angle and focus on him being a powerful let creepy warlock who highlights the mastery that magic can bring; a freedom from want and responsibility.
The Willow goes too far, hurts some people (both deliberately and not ala Tabula Rasa) and/or finds out some horrific truth about Rack's use of magic/the source of his power and swears off its use; either in an existential crisis kinda way (who even am I without the magicks?') or simply because her life (and the lives of her friends) is falling apart and this is her (somewhat simplistic) solution (cut the bad out of my life).
Seems like a much better way of illustrating the pitfalls of young adulthood; thinking you know it all but still screwing up like your parents did, but without the after-school special feel that Willow's arc otherwise has. I always thought that her 'magical addition' feels like Beer Bad only polished up and spread over multiple episodes as a subplot.
