Nickel is a very common metal across many minecraft tech mods throughout the ages, and should probably be included among the standard set of metals that come with Cotton Resources.

As for invar, I'm not sure if it should be added.

Closing this ticket since Cotton Resources is pretty much a discontinued project from the time being - feel free to reopen this if work continues

I don't see a reason to. We have plenty of other metals that cover the niche of nickel and invar, and "other minecraft mods have it" isn't really good enough for us.