What plots were left unresolved or forgotten in the OT?

[sdp]

Theres metalhead that was turned good and they mention he'd be a great ally against the shredder and we never see him again except for a cameo cleaning the lair one episode. Why set up something to never use him again?

Of course there is Usagi, who they had a great way to bring him back to his world at the end of the episode and they don't. Were there plans of a spin-off or something? weird they didn't show him again either. I know there was a space usagi proposal but that wasn't a spin-off from his OT appearance.