AGDI's KQ3 Redux already has
achievements of a sort. You only figure out which ones you got at the very end of the game.
As for adventure game achievements, I'm a bit conflicted. I really like achievements, but I just don't think they work well with that genre.
At their best, achievements are a goal to work towards. So when you're playing the game, you're both trying to win and trying to reach all those goals. This way, they can also point you toward experienced you might have missed otherwise. In an RPG, you might have achievements associated with optional side quests, or with maxing out your level, or with acquiring unique equipment, for example.
That sort of thing would be tricky with adventure games, though. There's generally just one story that every player ends up following. The only way to play it "better" than another player is generally via a better solution to a puzzle, or via doing collection things that are irrelevant to the plot (e.g. getting the assorted treasures in KQ1 and KQ2). It would also be extra important not to give puzzles or plot points away in the achievement descriptions, while still telling the player enough that they know what they need to work towards. Finding a balance there is difficult.
Also, it's in the nature of adventure games that a lot of achievements would be missable. Adventure games tend to have many points of no return (e.g. some world-changing event happening or you moving to a new area) and one-time events. That means that, if you didn't get the achievement at the time, you can't go back to get it later. At the same time, however, adventure games don't generally have replay value (aside from playing again much later). Playing the game through again immediately after winning is generally no fun, but playing the game super carefully (probably with a guide open) to make sure to get the achievements the first time through also isn't fun.
Some games enable achievement hunting by either giving you quick access to replay certain missions or by just opening up the world as sandbox once you've won, but I don't really see those working for adventure games.
The alternative would be to just have only achievements for actions that are required for winning the game. So the percentage of achievements gained would roughly correspond to the percentage of the game played, and winners would always have 100%. For most types of game, that sort of thing is very unsatisfying (although still better than no achievements at all), but it might be the best option for adventure games.
