Thanks, I'll take that into consideration. Honestly, I'm not sure where the stages should be. I've been thinking of alpha as time to add most features after the core systems are setup -- based on alphas not be available to the public or for dev testing only in general software. But perhaps I'll wait until I've add most/all mobs and fixed really glaring bugs (like unmeshed overhangs) -- EDIT: Perhaps I should at least have caverns in a game with "caverns" in the name first...
It's always difficult, my own perception of alphas would be somewhat complete, has the general gameplay loop, but still requires more features and still missing a lot of the final polishes. Beta is usually adding the polish and rarely ever new features. Prototypes, which is how I see your game personally, would be lacking most of the core mechanics and serve as testing smaller parts of the game.
It's also possible that I might be overestimating what the final product would be and you are aiming for something smaller. Which is good, I can only really get an understanding currently from the description of your game and what I played.
Regardless of what you choose to do, best of luck with it and hope it does well for your game!
My plan for this wasn't much more complicated than this, though I put a list of things I'd like to do with it in my latest update dev log.
At one point I did have the idea for an elaborate action RPG resembling a cross between Daggerfall and Minecraft with elements of shooters (Hexen) and even The Sims (with NPCs) -- but I'm sure all that is way beyond what I can realistically do. Anyway, this is not planned to become that!