In other words, you want the twin town of Ironport-Splintertown. Read on...
First, some background: As outlined in Forge of War (p. 17), the Mror Holds were the first region of Khorvaire to officially declare themselves independent from the Five Nations - all the way back in the year 914 (84 years before the present time). Karrnath was rather upset about this, but lacked the resources to do anything about it at the time. To take some of the sting out of this, the Iron Council (the leadership of the Mror Holds) gave Karrnath Most Favored Nation Status and wasted no time in selling them weapons, armor, and other war materials, assuring that they would "maintain close military ties". However by no means did the dwarves wish to lose out on the very profitable Southern Trade with Breland and Cyre - thus, they clandestinely hired on Lhazaarite "merchants" to smuggle their products, maintaining plausible deniability with Karrnath.
However, now that the Last War has ended the Mror Holds do no longer have to be clandestine about the Southern Trade - after all, everyone was now at peace and thus free to trade with whomever they wanted to. Naturally, this has cut into the war profiteering profits of the sea princes. However, they managed to bargain their way into the Treaty of Thronehold, and are now doing the whole "We are a verrrry respectable nation with verrrry serious businessmen and verrrry vigorous and innovative forms of taxation! Arrrr!"
In other words, their erstwhile business partners have become a major pain for the Mror Holds - any goods shipped from the north has to pass through pretty much the entirety of the Lhazaar Principalities, which means that every sea prince on the route will want either hefty bribes or plunder their goods outright. Thus, the Holds need a new trade route that will circumvent as much of the Principalities as possible. Fortunately, there is a possible route, though it will require some work.
Two years ago, the nations of Khorvaire signed the Treaty of Thronehold, which nailed down the national borders after the Last War. While the main focus of the extremely intense negotiations was on the borders of the original Five Nations with their neighbors, the newer nations had their own arguments - one of these being the boundary between the Mror Holds and its neighbors Q'barra and the Lhazaar Principalities. In the end, the Mror Holds managed to gain a small strip of coastal land that represented "a natural extension of the Ironroot Mountains" and thus clearly part of the Mror Holds (which was news to the Jhorash'tar tribes living there, but nobody invited them to Thronehold). The dwarves tried the same argument for the territory south of the Hoarfrost Mountains, but the sea prince of Cliffscrape considered the numerous fishing villages dotting the coastal plains to be part of her domain and objected most vehemently. The negotiations with Q'barra were easier, since the latter country did not have much in the way or resources to spare for this strip of no man's land this far north anyway. In the end, the current borders were worked out as a compromise in order to present a united front to the older nations.
Half a year later, the dwarves surveyed the land for a possible location for a new port (with the assistance of Lyrandar experts - the Mror Holds had excellent engineers, but little in the way of maritime expertise). It was soon determined that the very best location for a new port was at the mouth of the Crystal River - which, awkwardly enough, was the boundary line agreed upon with Q'barra in the Treaty of Thronehold. Furthermore, the site already had a small settlement named "Shardtown" by its inhabitants - a combined fishing village and hideout for smugglers who during the war specialized in lightweight/high value items - including Dragonshards.
Officially, the logic of nautical engineering won out. The dwarves arrived with a force of engineers, guards, administrators on the north side of the river mouth and announced to the startled inhabitants their intention that they intended to build a new city on the spot entitled "Ironport" (after the Ironroot Mountains). Anyone who wanted to stay under the new rulers could do so and take part in the many exiting and entirely legitimate business opportunities. Those who were unhappy about the regime change were politely but firmly pointed towards the other side of the river, where the settlement retained the name Splintertown and doubled down on the smuggling and all sorts of other dubious activities (many of which take advantage of the riches generated in Ironport).
Very privately, several Mror clan leaders see this problem as an opportunity. They would dearly like to grab more territory for the Holds (and their clans), and considering that Q'barra is weak, underpopulated, and focused on the southern parts of its territory, the country looks like a perfect victim. However, none of the other major nations are willing to allow such a blatant land grab and violation of another nation's territory so soon after the Treaty of Thronehold was signed. Thus, the Mror Holds need some sort of pretext for expanding southwards. Their strategy is as follows:
- Tolerate the presence of Splintertown, but subtly encourage it as a den of criminality and vice that causes ongoing problems in Ironport (frankly, this isn't hard) - and then complain loudly to the government of Q'barra, the worldwide press, and anyone else who will listen about it. Q'barra of course does not have the resources to "clean up" a town so far from its core settlement, but it might send the occasional lawman north who will predictably get horribly murdered in short order (either by the crime bosses or deniable agents of the dwarves). Eventually, the dwarves might be hailed for invading the town and "cleaning it up" by world opinion...
- Provoke some sort of attack or raid on Splintertown that is unlikely to threaten the defenses of Ironport but represents a major threat to the inhabitants of the smaller town. This can provide an excuse for the trained soldiers of Ironport to march forth and defend the town, since Q'barra is obviously unable to defend its people. And once the threat is defeated, the soldiers might stick around... and the inhabitants might cheer them for it.
(Side note: For an alternative campaign, consider making the PCs the "lawmen" sent north by King Sebastes in order to clean up Splintertown and then discover all the... complexities of their assignment once they arrive. Assuming that they won't get horribly murdered first.)
But these are far from the only issues facing Ironport-Splintertown. To the north, the Mror Holds need to establish a secure route through the southern Ironrood Mountains, which means dealing with the orc tribes one way or another. Some clans are in favor of giving the orcs a place in the Iron Council, while others are bitterly opposed and would like to see nothing more than have all the orcs wiped out - and they are perfectly willing to send agents sabotaging each others' efforts in this. There is also the question how this new region is supposed to be distributed among the clans, since it is not part of their traditional territories (see the Player's Guide to Eberron, p. 55 for a map). For Ironport and the route leading to it at least, the Iron Council has agreed on a basic profit-sharing scheme based on the amount of investments each involved clan has made. Currently the lawyers and accountants are battling over the details, and whether this agreement will survive the next round of renegotiations in a recognizable form is anyone's guess. Meanwhile, beneath the southern Ironroot Mountains things stir in the depths of Khyber, and duergar have been spotted in increasing numbers. How this will affect the new trade route is anyone's guess.
To the west, the hill country gradually gives way to the Talenta Plains, and dinosaurs are a common sight. While the halflings themselves are good neighbors, and their trade caravans become increasingly common as Ironport grows, the Valenar raiders range far and wide from their southern lands, and eventually the growing town might represent too tempting a target for them.
To the south, the lizardmen and dragonborn of the marshes seem mostly quiescent, though the occasional group of dragonborn pilgirms passes through town on their way to the Boneyard. But the swamps hide numerous brass-embellished ruins from the Age of Demons, and the explorers of Ironport-Splintertown might not be wise enough to stay away from their sleeping inhabitants. Indeed, perhaps agents of the Lords of Dust are already active in town in order to encourage such explorers...
To the east, the Sea Princes watch the growth of Ironport with fear and greed. The northern princes fear the inevitable loss of income - and with it, prestige and political power - that will result once the Southern Route is firmly established, while the southern princes hope for increased opportunities for bribes and plunder. Prince might turn against prince, and this issue is what might cause the young nation to splinter again in its component parts. For now, the alliance holds - but for how long?
And to the far east, the Inspired watch this new development with interest. Any new major port on the eastern end of Khorvaire represents another opportunity for infiltration from Riedra - and once they realize how explosive the political situation is, they will do their best to cause the situation degenerate further while presenting a helpful and friendly face to everyone.
It is in this maelstrom of political intrigue where the PCs will make their names. Will they just focus on plunder and profit to their own benefits? Or will they make a stand and shape this new town in accordance with their vision?