Heh I do some shipping myself. I like the concept of post game, too. However, I am often not comfortable with roleplaying romance. Not my thing, sorry. I'm sure you will find someone on here though. Tons of people are cool with romance and/or shipping.
Also: Noticed you just joined, so welcome...
It depends. Is it a playthrough, a post game, a romance novel? Many people compromise on plots anyways, so list what you want to do and someone will come along who wants to do what you want to do, almost exactly usually. This is where compromising comes in, as well as talking to each other to...
Hello! While I am a fan of undertale myself, and may consider roleplaying it sometime, many people here and in other places would prefer you to be more clear and detailed on what you would like to do. If you could elaborate, or even just list possibilities, it would greatly enhance your chances...