Say, for Diplomacy, a Holo-Diplomat (maybe with a unique ability).
Or science, some form of Holo-Research Lab Analyst.
Or for Colonization, a Holo-Quartermaster with a unique ability that is themed around what he's designed to handle.
For Operations, a Holo Sensor Tech with a new space ability.
The idea is to reward players in a unique new way for playing the doff system enough to get these holo-doffs, with the Tier 4 version being something terrifyingly powerful as a reward for working so hard to get them.
And, even what would be otherwise ability-less doffs would receive some new power around their chosen role, not overpowering, but a new option for how to play, and flavorful in their own quirky way (trader space ability reduced buy costs, extra sell percentage?; development holo-adviser Doff gives a small per-hit damage buff from known tactics and weaknesses records? Holo-councilor is the same, but for the flipside of combat?, etc etc...)
Holo entertainers are already in the game, you get them from a critical success on the "Special Deal" missions. These are indirectly linked to trade rank in that the rarity of the special deal mission determines the rarity of the DOff and you need to have a certain trade rank to get the mission.
Rank 1 - Common Mission & DOFF
Rank 2 - Uncommon Mission & DOFF
Rank 3 - Rare Mission & DOFF
Rank 4 - Very Rare Mission & DOFF
Eh, some kinds of holographic DO's make sense and others don't.
I'd like to see other kinds of distinctives for the Commendation categories that don't have holograms or other benefits like Diplomacy and Marauding do.
For example, I'd like to see Transwarp abilities with different locations than the standard set. Like Colonial allowing transwarp to exploration clusters, or Trade opening up transwarp locations to other worlds (such as Memory Alpha, Drozana Station, etc.). They would complement what we already have and provide an alternative for those choosing not to level up Diplomacy or Marauding.
Or maybe trade allows free travel like Diplomacy and Marauding do. Or lets you summon a cargo transport.
But I can see too many kinds of holos being too much of a good thing.
I definitely could see a need for diplomatic holograms, as well as piloting holograms.
As for holograms going on away missions, remember that a lot has transpired since The Doctor was stuck in Voyager's sickbay. Not only did he obtain a portable holoemitter (which, according to at least one Episode I've played here, are now reasonably common in the Federation and Klingon Empire), but he was also trasmitted and returned via subspace at least once.
That said, I think this should become a limited thing. Let us collect, craft, or replicate portable holo-emitters (I favor crafting), and allow one off-ship hologram per emitter. Not included in this limit would be missions to certain major locations such as Memory Alpha, Earth Spacedock, DS9, DSK-7, and other locations where the hologram can be transmitted and returned via subspace.
(Could shuttlecraft have holoemitters that holograms could use? Sure, but the Assignment would have to be one where the hologram wouldn't be required to leave the shuttle, such as transporting prisoners and colonists.)