I've just changed into the per-user pricing (From fixed rate) to give better flexibility for organizations of all sizes. Would look into the pricing again in 6 months based on how it performs for now.
I am always tempted to make a slack app. Not this particular idea, likely something else.
If you're looking to build in any marketplace, shopify is more strealimed as majority of the shopify's users spends on the apps (and the billing and stuff is integrated) compared to any other marketplace.