tinyE: Of course, you could always get an emulator and play Metroid
and Castlevania.
Castlevania isn't a metroidvania, so it won't work (though some of the later sequels will).
Metroid is not a good choice if you're worried about where to go next; I would suggest starting either with Super Metroid or a game that came after that. Metroid Fusion is heavily linear and guided (to the point that long-time series fans have criticized the game for that), though eventually comes a part where you have to find your way out of somewhere without tips.
dtgreene: ... Guacamelee is a nother linear-with-optional-backtracking metroidvania, this time focused on melee combat (including the ability to throw enemies) and without RPG elements. Some of the platforming can get difficult, and there are many moves thar require pressing multiple buttons simultaneously that are required for platforming. This is the sort of game where you pretty much *need* a game controller to play comfortably; playing with a keyboard might be possible, but only if that keyboard doesn't have an issue with lots of simultaneous key presses...
Mafwek: Guacamelee, as I recall controls like a fighting game, and fighting games objectively are easier to play on keyboard rather than controller. Other than that, correct assessment.
One problem, however, is that (typewriter-style, not piano-style) keyboards are often limited in how many simultaneous keypresses, and which sets of simultaneous keypresses, the keyboard will recognize at once. As a result, many keyboards will not be suitable for Guacamelee, as that game does require you to press many buttons at once at times. So, even if one might consider it easier to play on a keyboard, that only works if the keyboard can handle it, and many can't.