Does anybody else see the possibility of that colony being Yu Dao? We've seen a bit of a look at it, the colony looks large, and it has a wall around it. Most of its surroundings are forest. Sure, a lot has change since then, but if you look you'll see that Yu Dao is large, and has a wall. It also seems to be surrounded by a lot of forest like surroundings.
Not saying anything else, other than this has been a sort of a headcanon and a personal theory.
The thing is, I don't think we know that for certain. I only read The Promise for the first time last weekend, and I don't remember offhand whether Yu Dao's exact location was ever specificied. I do seem to recall that Republic City is located in an area of the former Earth Kingdom in which a large number of Fire Nation colonies were located, though.
It is almost guaranteed that Republic City was built on the site of one of the oldest Fire Nation colonies - which include Yu Dao. Whether it is that specific colony, I am not certain we have firm confirmation one way or another. I will have to double-check to see if The Promise ever shows exactly where Yu Dao itself is located.
Looks like it says on the Yu Dao page that it was the first colony established. I don't think its precise location has ever been definitively confirmed—and it looks to be inland, so it's likely actually not Republic City proper—but it definitely became part of the URN.
Yeah, I don't recall ever seeing Yu Dao's exact location confirmed. If it is inland rather than right on the coast, it's probably not very far inland. And I would think that you are correct and it would be inland, since we know the Fire Nation army arrived by sea, but at Yu Dao proper we never see any ships, only ground forces, tanks, etc...So I think you might be right. Definitely part of the URN, of course, but it might not be RC itself.
See, I re-read the Promise over the weekend, and I am relatively certain that Yu Dao is not where Republic City itself is located; the terrain looks too different, and while it's clearly not all that far from the coast, it is just as clearly not on the coast, since the Fire Nation had tanks and komodo rhinos, which travelled a substantial distance before arriving at Yu Dao. If YD were in RC's eventual location, Zuko would've just rolled up with his fleet and disembarked directly, without the need for a substantial overland journey.
Based on what we've seen in the comics, I don't think Republic City has been built yet. I think the land where the Earthen Fire Refinery was built is the site of the future Republic City based on the terrain and other hints in the plot. I reread "The Rift" trilogy and the land the refinery is located is surrounded by mountains, and it in turn the land surrounds a bay (like RC). Also, Aang mentioned there was a small island directly off the coast where airbenders used to fly kites (maybe where Air Temple Island later is?). Furthermore, I bought the three-in-one edition of the trilogy with author's notes, and one author makes special note of how the refinery's guards wear the same armor as the RC metalbending police (heavily implying there is a connection).