I had to check up on this. Yeah, it's true. The WN on Soyetsu is a mere 40 chapters, and the official LN is only one volume. Looks like the last chapter of the WN ends at the exact point this chapter of the manga ends. Dunno about the LN, but I can't imagine that they added much more to it than that.
WN:
https://ncode.syosetu.com/n8990io/
LN:
https://www.kodansha.co.jp/comic/products/0000404283
Agreed, it is a shame that it ended this early on. There's still so much potential for the plot, not the least of which how his relationships with these three very different individuals goes. (And then there's the possibility that his tamed slimes could "evolve". Slime-girls, anyone?) Granted, it's kinda hard to top the taming of a dungeon boss, which is why I think the original author decided to end it there - he probably felt that he didn't have the creative energy to push any further than that without the story getting bogged down or repetitive.
But there are real-life monsters that can pose a threat. Namely: corporations. IIRC, corporate sponsorship of adventurer parties is a thing in this setting, so that could become an issue.
Another good issue would be getting Elvaana some sort of civil rights.
Oh, hey! Maybe those adventurers that were going around raping and killing other adventurers had ties to the Yakuza.
All kinds of possibilities. Ah well. Dem's da breaks.