We get a lot of sequels in the gaming world, and a common criticism is when a series isn't really innovating enough. We're given an open world game that takes 40 hours, with DLC stretching it out 20 more, and see a sequel releasing that cut out it's late 30 hours because players were already getting bored.

Meanwhile, there's some other types of games where any addition in the form of "It's just more levels in the series" is perfectly satisfying. Often, this is a hard measure to replicate since these types of series often demand the creators are very inventive and detailed with their content - this likely wouldn't be a matter of rearranging tiles in a level editor to present a very slightly different situation.

What I've often seen is that such games will add incredibly small, insignificant "New Gameplay Features" just so they have something to put on the back of the box, but that tend to be easily forgotten in standard play (yet, the game as a whole still ends up being fun).

The specific series that come to mind for me with "Level-driven games" are:

Hitman - the way the levels are made naturally necessitates some creativity both from the level makers to come up with unique foibles and weaknesses to each target, and from the players to discover both the intended and unintended methods of elimination.
Ace Attorney - While they series has come up with various magical/unusual methods for pointing out contradictions in court, the appeal is still in the mysteries themselves, and it's never needed much beyond the basic gameplay, and the incredibly detailed and well-animated characters to hook people in.
Half-Life - For its time, anyway. While its Episodes certainly made efforts to present new features, quite often the star of Half-Life games isn't really in any core features or gameplay mechanics, but in the inventive designs of its levels, tied in with a penchant for environmental storytelling; making you feel the world was more than an arrangement of blocks and paths. For a long time, the wait for Valve-made episodes was alleviated with modder-made levels hoping to approach the inventive qualities of the original games.
Yakuza - While the series has undergone a major overhaul moving to JRPG combat mode, for 6+ games it satisfied a simple formula: Dramatic stories driven by cutscenes, as well as a huge variety of mini quests, of boundless variety and very low logic. For many of their games, they weren't doing a whole lot to re-contextualize their core gameplay, being fisticuffs combat, and it still worked out well (plus, they're continuing to go that route for games like Kiryu's last game)

To open up discussion, and put the question as simply as I can: Which games do you follow, that you wish could be eternally supported by their devs, by simply continuing to release new "level packs" or their functional equivalent, with no need to revamp gameplay formulas?