meaning of の after すごい


> せっかくならもっとすげえ **の** 用意してもらいてえもんだな

why is it there?




の turns an adjective into a noun, taking it from "amazing" (すごい) to "something
amazing" (すごいの).

If we take your sentence and simply write it in a more standard way, we get:
せっかくならもっとすごいのを用意してもらいたいものだ。

_せっかくなら  
_ _If you are going to go out of your way,_

_用意してもらいたいものだ。  
_ _(then come on...) I'd like you to prepare_

_もっとすごいのを  
_ _something even more amazing!_

See: [Particle の (Nominalizer)](https://www.tofugu.com/japanese-
grammar/particle-no-nominalizer/)

