I also have a recommendation. Don't add a tutorial, having the player figure every thing out is way better. like the feeling of figuring out that the big electric tank robot has a panel on its back you can open and shoot. That was cool.
I want to have some tutorials. Although everything could be figured out by some people eventually, I'd still want to present some basic concepts. Energy management, crafting etc. Enemies and anything you may encounter inside? That's for the players to figure out :) The way I add new NPCs is that each one has to have something unique to it. And some of them may have quite peculiar features. An example: the shield mushroom you can find, can be used as a shield of course. But also it can be used as a medkit to replenish your health when it goes low. And you can refill it.