Has anyone heard if they are planning to release yet more campaign material for Tier 3-4 continuing the DoIP storyline?  Or are the three expansions the last that are planned?

There's been no mention of it.

Since the vast, vast majority of campaigns never reach those levels, I would be surprised to see that content created here.

Couple of things, though:

- I would not be surprised to see DMsGuild products that continued the campaign through level 20.

- I would not be surprised if DNDBeyond added more DNDBeyond-exclusive adventures (not related to DoIP) to the marketplace - unless their license with WoTC prevents them from doing so.

Cool.  I think if I up the challenge on Rise of Tiamat it could make a good Tier 3 epic to build on DoiP given the presence of so many dragons in the current 4 adventures (esp. if I combine the first part of DoIP with LMoP).

Quote from AbessiaMalktho >>

Cool.  I think if I up the challenge on Rise of Tiamat it could make a good Tier 3 epic to build on DoiP given the presence of so many dragons in the current 4 adventures (esp. if I combine the first part of DoIP with LMoP).

 Agreed. Another option would be to use levels 13-20 of Waterdeep: Dungeon of the Mad Mage.