What support for Ultimate Campaign do you want 3pp to do?

I am farily happy with the support that kingdom building gets with Book of the River Nations: Complete Player's Reference for Kingdom Building and Ultimate Rulership

And wondered for those customers who have read through it, if there was anything third party publishers should look at doing?

A Book of Armies.

Also, there's plenty of realms to be explored with the downtime system, mass combat spells (or magic items), or a PDF of tile sets for city building, fleshing out the background generator for non-standard races (such as those from the Advanced Race Guide) and more background options for classes, Story Feats (especially for those 3PPs with campaign settings of their own), building events, a nice thieving guild system using the Contacts campaign system...etc. and the such. :D

What Lilith said

A revised Downtime system that my players can look at and not break from just looking at it funny.

I'll tell you what we need: A Legend of the Hero Kings for Pathfinder.
LotHK was a sourcebook for Birthright (clear inspiration for the kingdom rules) that had short adventures based on many of the random events that happen during the rulership of a domain.

Someone, or many someones, should design a book or collection of adventures that are specifically keyed to events on the Event Phase table.

The best way possible to keep such a product relevant is to make the adventures as open and modular as possible, with sidebars for how to increase or decrease the expected level of the adventurers, i.e. CR 5,7, and 9 versions of the villain, sidebars on how to add additional threats to up the CR of specific encounters, etc.

Expanded rooms, expanded building test tables.

Rules for slowing down the downtime some for games that have a lot of downtime (IE: Downtime by week instead of day).

Rules for doing the some of the downtime while 'on the clock' (similar to crafting on the road) where you get some credit for on the road stuff (where it makes sense, like retraining, etc). More expensive and time consuming, but doable outside of downtime for those games that have little or no downtime.

Expanded backgrounds for alternate realities (realities with guns, realities with psionics, etc). Additional balanced traits and drawbacks.

A Book of Armies.

This. Though Legendary Games, who did Ultimate Rulership, will also be doing Ultimate War to expand on the mass combat rules.

That said, if someone wanted to do something for PC backgrounds and families for the non-core races I'd snap that up in a heartbeat.

Rooms, teams, buildings, army abilities, and so forth to support 3PP classes.

Something that takes the downtime rules for building buildings and the Kingdom building rules and combines them so you can add onto and expand large buildings like castles, palaces, or noble villas within the kingdom building rules.

Lilith wrote:

A Book of Armies.

This. Though Legendary Games, who did Ultimate Rulership, will also be doing Ultimate War to expand on the mass combat rules.

Yep. On track for an early July release.

Here is a crazy idea, how about getting two or more 3PPs to work together on a project like Green Ronin and Paradigm Concepts did with their OGL Interlink products. How about that?

I'd love the link-idea! :D

I imagine you could really go all-in for kingdom building and war by having class archetypes that offer bonuses to those subsets, plus Prestige Classes.
Which would pair nicely with class features designed around nobility and courtly intrigue.

So the game entirely focused on running a kingdom, war, and politics has class options. Where you're not playing adventurers but rulers.