Is there a way to create a new pact type (not patron) for warlocks?  If not, are there any plans to add one?

I'm creating a new patron for my world, and would like to create a new pact type to go along with them, adding some new invocations and mystic arcanum to go along with them.

Those are all base class features. We cannot affect any of those.

I think that falls under Subclass. You can create a new Patron type. Which includes the Expanded Spell List and other Patron features.

I hope this helps.

Warlocks have a patron (archfey, old one, fiend...), and a pact type(tome, blade, chain...).  They basically have two "subclasses".  We can make new patrons, but not new pact types

A Pact is not really like a subclass, it’s more like their invocations. It’s a class feature. 

A class feature that plays a big part in how you play the character.  New pact types would be an awesome addition to the homebrew tools.