I love card games and I LOVE your concept, it's simple but effective!
I think that it could also work for a board game which is nice.
The way you develop the "risk" theme is also clever, but I think it needs something to make the choice a bit more "strategic", maybe knowing how many and which kind of islands you still have to discover (without knowing the order of course) could be a nice add.
Anyway well done, the concept really inspires me
and... if you try mine, let me know what you think!