I wonder if Bryke will make irosami a thing in book 4 after all? They apparently thought about it in book 1 and most of us agree that the Fire Nation will come into play next season, so Iroh II has got to play a part, right? On the other hand Asami is usually the one drawing the short end of the stick, so I wouldn't be surprised if she stayed the only one without a love interest in the end. But I can hope for Varrick to wear Fire Nation clothing, right, that's reasonable? He has 2 out of 3 now.
I’m going to be honest, I’m not super confident in Irohsami’s prospects of becoming canon. Iroh’s age has never been solidified, but as a high-ranking officer, he’s not going to fall within the same accepted peer group as Asami and the others. Relationships with age gaps aren’t unusual by any means, not within the real world, and certainly not within the Avatarverse (Pema and Tenzin have 15 years between them), but the matter becomes a bit more loaded the younger a character gets. I’m not sure if the story wants to deal with the implications that arise with this sort of romance given the respective ages of the characters.
From what I recall, Asami joining the United Forces was a serious consideration, but her running off with Iroh was proposed as a joke pitch, and was centered more on how little Asami and Mako knew about each other than Iroh and Asami’s compatibility as a couple. It doesn’t mean Irohsami couldn’t happen or shouldn’t happen, but I’m not confident enough to put weight on a prediction one way or another.
As a minor tangent, I think it’s worth adding that not necessarily every character needs a romantic conclusion. It’s important to be mindful of whether you’re pairing a character off for the sake of pairing them off, or if the relationship adds something to the character’s development and range of conflicts. For instance, Lin Beifong will probably never have a canon love interest, and that’s alright, because that’s not what her character is about.
We do know Dante Basco has recorded lines for Iroh since Book 2, so I’m sure we’ll see him again— especially if the royal family has any sort of role in the future. And I definitely think we’ll see the Fire Nation at some point, seeing as it’s the only location we’ve yet to visit. We can certainly hope Varrick will tag along, too; seeing him in Fire Nation clothing would be A++, I heard they make the best red stuff there.