You can... unless you are violating regulations/laws. Ex: trying to make money (or even using) copyrighted material without the owner's consent. Meaning no subscription in fangames.

Actually, that is fairly inaccurate in a lot of cases.

True enough, you cannot charge people to play a fangame, that is illegal. However, it is entirely legal to charge people for any bandwidth or server space they are using to play a fangame, or any type of game.
The same applies for any stolen resources/code or resources/code which you have not been given permission to use in such a way.

It might sound silly because in effect it is just the same as charging someone to play a game that is illegal. But as far as the law is concerned there is a major difference, you simply have to make it clear that people are not paying to play the game, they are paying for bandwidth/server space used while playing the game.

In much the same way, you can accept "donations" for a game, regardless of whether that game is illegal or not. Provided these donations are not required to play the game that is (though it is fine for them to provide in game bonuses).

In a similar way, you could get away with selling ingame advantages. This is simply due to the fact that these ingame advantages are in no way property of the original copyright owners of whatever you have stolen/based your game upon.

Though a fangame is illegal without permission, provided you watch your wording it is not illegal to make money off of it. Though this method of making money has to be in an indirect method that is not related to the legality of the game.

Indeed. Charging for bandwidth? No, you're effectively charging to play the game, and you can't play the game without paying. What you do with the money afterward is an unrelated matter.