Sequels that share or build on characters, plot, and so on with their predecessors are definitely derivative works. (Let's ignore the literal sharing of assets and bits of code that's also common for sequels since that's obviously more straightforward, and not strictly necessary.)

Trademark definitely comes into play, and is probably the more important form of IP here in practice (and for some series may be the only really effective protection). But you still couldn't go out and make a game with Master Chief or Nathan Drake even if you called it something other than Halo or Uncharted.

Trademarks are also interesting and somewhat unique for video games: the typical rule is that the title of a book or movie, etc. is not eligible for a trademark. You need a name that's being used for a series to get a trademark. But video games are excluded from this rule and can (and almost always do) get trademarks for their titles even when they're standalones.