/sigh
We're talking about Minetest-engine, or are we talking about Minetest-game? Minetest is bigger than just being a
Mine-crack clone- it's a flexible engine that can be used to make a variety of games.*
But, let's say we're talking about adding
Minecraft features into Minetest.
An armo(u)r mod exists, but hasn't been merged into minetest-game. Reason? It's pretty useless in singleplayer, and server admins can easily add it if they want.
Character previews/third person has been discussed to death. The current consensus is "Cool feature, but I'm not going out of my way to make it happen".
Are we talking about skins? Skins with a limited preview functionality have been
created as a mod. Again, it wasn't added to minetest-game because it has no purpose in singleplayer.
tl;dr:
Ain't happening soon.
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* Yes, you can easily make a First Person Shooter/Arena game.