One of the surprisingly better "That Time I Got Reincarnated as a Slime" spin-offs, might even be the best. "Tensei shitara Slime Datta Ken: Clayman Revenge" continues off of where Clayman was killed by Rimuru. However, Clayman awakes to find himself several decades before his death (70 years) and discovers that he's in a limited time-loop due to an ability (it is unclear if it's his or someone else's). So he quickly devises long-term plans to avoid incurring Rimuru's wrath and ultimately avoid his death. Now this might sound like one of those many "Villainous Isekai" mangas out there right now (which it kind of is) butthis came out before the trend and is far better paced with much better characters!

This spin-off, in my honest opinion, might be better than the main manga (and light novel) itself and is far more enjoyable.

Clayman already has his established entourage of characters both previously shown up in the main story and those who has not appeared yet. All of them are much more likeable, adorkable, and lighthearted compared to their appearance 70 years later during the main manga. Clayman himself is lot more relaxed and lighthearted compared to his more arrogant, scheming future self as well (there is a reason for that as it's very quickly hinted at early on in this manga and will later be revealed in the main story).

Clayman's main plan is to simply build up strength, not just his strength but his subordinates as well. During this time, he interacts with and recruits many characters some of which are characters that he would later associate with or even ancestors/parents of future characters. Potentially changing future events towards his favor.

And with that, this spin-off is already doing much better in 'building-up' Clayman's army and nation than Rimuru ever did in the main manga as it's spread out with minor time-skips and natural planning and interactions with other characters to work in Clayman's favor. Rimuru on the other hand gets handed victories, powers, & people left and right just through brute-force & unrealistic generosity and all of Rimuru's interactions are basically "I'm a nice guy, work with me!" (Clayman does this to some extent as well) and all of that happens in a very short span of time.

LN readers already know but it's a clear case of quality story-telling difference in writers as Fuse isn't writing this spin-off, it's Wataru Kajika.

As the main manga continues to falter and lose momentum, this spin-off continues to reel me back in and I hope it continues to keep me engaged with all the fun characters it keeps pulling in and the promise of a much better future for Clayman.