Adding well known characters without permission would break the copyright law, and could end in a lawsuit.
How about making cameos from other HRPG or Ero-game? Like they got lost for a moment and then they ended up to this game dimension because of casting wrong magic spell for teleporting, so then you got a simple quest to help them to where they came from.
I think it would be a cool collaboration, but it's just a suggestion tho, and maybe it will be hard for you to contact other games developer and maybe they're not as humble and friendly as you, Ryen. but I do hope you can make it in the future.
For the other things, damn, you did a fantastic job. Keep it up you horny dragon blood!