Featured upon full crystal
I think it'd be a pretty cool idea if all streamers got a crystal, but only partners got the $$ tiers. For non-partners, filling up the crystal would instead make them eligible to get featured somehow. Whether that feature is the actual featured, or up and coming, or channel one, or channel two. This would help give sparks a point to non-partners and also give them an avenue to help them grow into partners.
You could also potentially add functionality to allow a partner to turn down the money on a tier in favor of the featured spot as well to help them grow.