I'd be interested in adapters to plug Super NES controllers into NES, particularly things like the Capcom Fighter Power Stick and Super NES Mouse. This would be an extension cable with a Super NES controller receptacle at one end and an NES controller plug at the other.
You could bundle a mouse, an adapter, and a multicart of homebrew mouse games (which ROM I could provide).
I'm going to be building some of those, as shown on here:https://www.kickstarter.com/projects/1115970826/spook-o-tron-a-halloween-themed-homebrew-game-for/description
I'm really preferring to have a factory make a bunch of them. I would definitely like to supply large quantities for cheap to homebrewers/publishers. But if I go that way I'll be stuck with a huge amount of them for a long time. In any event, I'll build some by hand but they won't be as cheap as I'd like.