Alternatives to Instinct e-Commerce plugin?

Some of my clients have used and profited from this plugin, and are now asking “what’s next?”

They typically started with 20 or 30 products, and now have hundreds. I’m not saying e-commerce can’t handle hundreds of products, but I am saying that in their minds, e-commerce is “beginner” software, and they want “pro” software. I want to give them what they want.

I know that what’s next is probably not a WP plugin.