While more crossovers and references would be nice, I think there are couple things to keep in mind.
- SFW developers are not going to collaborate with NSFW developers. In your example Sunny Syrup would be risking a lot on their reputation having their character portrayed in this way.
- There maybe scope for crossovers with other NSFW developers, but this is normally quid pro quo "you can use my character if I can use yours." So you are more likely to see actual characters from other games where Kal has working relationship with said developer.
- You most likely to see (and there already is) crossovers from other Arvus Games like Roundscape, Below Sunshade, Arenus...
- That is not to say Kal cannot take inspiration from other games when creating characters and events though... so you never know...