Hey, I've got this idea down for an intro to a thrash metal song... I like it but I'm not sure I like the time signatures... just seems out of place to go from that straight into 4/4 Should I keep the intro or bin it /change it up?

Why don't you keep it in 12/8? That's what I would do. Some different meters are never wrong. Keeps it more interesting.
Well, it seems fine to go into 4/4, this is just something I tried to test it, I know it's kinda not of the genre you wanted, but it works. (:
idea edited.gp5