meaning of の after すごい

Question

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?

Answer 1

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)