so this one is a tough one to describe, but basically, games that aren’t officially part of the same series, but it’s plain to everyone capable of critical thought that they are.
the classic examples would be kid kool, psycho fox, and decap attack. (coincidentally, kid kool and decap attack are also both licensed games that got reskinned and delicensed in the west, though the two licenses were also both unrelated to each other). all three games share a lot of mechanics and quirks: heavily momentum-based movement, weird stretchy punches, those big bendy poles you can jump on, and so on.
another well-known one would be gunstar heroes, alien soldier, and gunstar super heroes. obviously, the gunstars are related, and there are a lot of similarities with alien soldier, but the thing that really links the three is the fixed/free shot, and the incremental improvment of the same:
in gunstar heroes, you pick one or the other, and you’re stuck with your choice for the entire run.
alien soldier improves on this by allowing you to change between the two at will, though this is its own action and requires the player to stop moving and shooting for a split second.
gunstar super heroes improves on it even more by having seperate fire buttons for fixed and free fire, as well as adding a third firing mode, in which the player can move around while continuously firing in one direction.
actually, due to the evolutionary nature of treasure’s game design, a lot of their games have some kind of mechanical or technical connection. like, the way the homing shots in dynamite headdy is very similar to the movement of the homing shots in the above games, for example. and the tiny toons gba game they made was essentially a shallow prototype for the excellent astro boy gba game they released a bit later
less well known to people who’ve never had to read me ramble on the topic are psychic force, psychic force 2012, and tv animation x: unmei no tatakai. unmei no tatakai isn’t just a reskin, or a game in the style of psychic force, it takes all of pf2012’s mechanics, some of its ui elements, and the basic concept in general* and again, it improves on, refines, and adds to them, just like you’d expect a sequel to.
*to be fair, though, the general concept and aesthetic of the psychic force games owes a lot to the original x/1999 manga on which the anime and game are based.
Illusion of Gaia 
