If I'm understanding this correctly, you want the rights to use this person's Intellectual Property (IP), with you as the final authority to OK how the IP will be portrayed in your project. I'm also assuming that if the creator decides to leave the project, you would want the right to contract other writers and continue the story.
What I'm missing is if you still retain the rights after the project is completed, but this is a lot of control you're asking for.
There are three options I can see at the moment:
- Buy all the rights
Since I don't know the popularity or the preexisting level of investment this person has made (physical, emotional or financial), it's difficult to know at what price point the creator will part with that amount of control over their work. If their investment is weak, you can get it for fairly cheap (Maybe $100-$500). If not, come back with a higher offer or start negotiating what rights you want to purchase. You will get to make all the changes you want and can pursue legal recourse if the original creator uses the IP they've sold to you for commercial purposes. If you plan on continuing to make more projects using this IP, it's something to consider.
- Buy limited rights
The creator will keep the rights to their IP and allow you to use it per project. This one has a lot more grey areas, especially since you want to make changes in the original IP. Royalties should also be discussed.
- Make enough changes that it becomes a completely new IP
Do you like the setting? Consider a wealthier/poorer area in a similar world. Is there a relationship dynamic you really like? Maybe shuffle around the relationships of side characters. Commissioning an artist to draw the characters can also differentiate your characters to the creator's enough that you can call it your own IP. It's more work, but it's also probably the cheapest and most frictionless option.
Whatever you choose, be clear in what you want and how much you're willing to pay. Research how much rights go for small indie writers and take some time to put together an offer. Know what issues you can't compromise and which ones you can.
(Also, commissioning the creator for further writing is a separate issue from buying IP. A writing contract is pretty easy to find, so I won't go into that.)
Tl;dr: You need to know what rights you want to use before you can find a contract for it.
