I'm pretty much in agreement with what is being said here already. A lot of existing solutions, like Carnivore2 and GR8NET already come pretty close. That's why they are always such an easy recommendation... to dedicated MSX users.
And that's why I feel the biggest possible next step, as others have pointed out, is in quality-of-life and ease of use features. I don't think the goal should be to make them huge commercial successes, like other flash cartridges for other platforms, but move toward a feature set so they are solid recommendations for new users and veterans alike.
So you'll have your basics, like sound extensions, RAM extensions and flash media as base. And on top of that, I would love to see things like:
- built-in menu for managing properties of cart
- loading ROMs straight from the cart's menu
- easy update procedure (from media or via USB port)
- line out for audio (on top of internal routing)
Most existing cartridges come very close to this! But I still feel they require more explanation to newcomers (myself included!) than should be necessary. 
