Doukutsu Randamu is a fan-made mod for Cave Story, which entirely overhauls the game into a format that's thoroughly replayable. Levels generate in a variety of ways, including randomized cave systems, rectangular laboratory squares, and sky settings - not to mention spacious and cramped variants. Different environmental elements also exist, such as flooded levels, areas with flamethrowers, or even terrain embedded with explosives or under a monsoon of falling blocks.
    Enemies are also rather varied to an extent. Many are familiar to Cave Story players, such as critters, bats, the bugs from Egg Corridor, and others. Some are modified with new abilities to make things interesting. This is true with bosses as well, many of which are from the original game, but adapted to the new format - as are some entirely new bosses. I would be curious about different types of boss arenas, but it works excellently as is so far. More difficult enemies show up later in the ten-floor runs in a consistent, understandable way.
    The item system is also fun to play with. Depending on the difficulty, the player is challenged to equip a specific limit of items at once. However, plenty of options become available throughout a run, which is further added to with a crafting mechanic. Finding rare Miracle Matter and bringing it to a crafting station allows for improvements, should the right items be found. However, I have occasionally cleared runs after finding three miracle matter, but being unable to craft with them. I would be curious about seeing more combinations to counteract this to an extent, so that these options extend more to crafting and creativity.
    (Oh, and there are two weapons that act as building tools! They effectively guarantee that levels can be dug through, climbed around, and completed.)
    Leaderboards are a good way to encourage a player to try to beat a previous score, or even go for the game's higher difficulties with their new challenges. The abilities to enter a particular seed in, choose from many different characters, or even set up custom configurations for a game further add to the replayability factor.
    The graphics and audio also do the same in that way. A variety of new GUI pieces, tiles, and the point counter add a unique appearance to the game, and the many audio tracks help greatly with keeping the game pleasant to listen to. Given that this is ultimately a fan mod, the use of popular songs from other games, combined with remixes of Cave Story songs, are an excellent way to not only reference them, but give players another format to listen to.
    Lastly, the modding compatibility seems promising, although I don't have any personal experience with it at this time. If there's content that you want to add to the game yourself, the Itch page has a link to modding resources. You should be able to add things such as unported Cave Story bosses (Monster X, Ma Pignon, etc.), or a gun that shoots spikes, or whatever other ideas come to mind!
    In summary, Doukutsu Randamu has a large variety of pieces that mix and match together incredibly well. Minor issues do nothing to harm its quality; levels are unique, the gameplay mechanics and settings ensure replayability, and the mod overall feels like a complete game. I can strongly recommend this to anybody who has played through Cave Story and wants to play more!