I personally do not see the need to introduce an extra building on Gondor settlements.
Halbarad has explained some of the reasons already.
Gondor has 5 different buildings that give you benefits (4 economy), but 4 of them are inside the fortress and only one is outside the fortress. Yes Angmar has 5 buildings with benefits, but at the End of the game they are not as good as Gondors buildings.
The thing is, that you can combine the benefits from Gondor MUCH better then the benefits from Angmars buildings: You always use buildings, so stoneworkers is always a useful benefit.
If you then build riders, you can use the farms to get them for much less money then normal and you can upgrade them for less money then normal. Same thing with your Tower Guards.
....
The marketplace is rarely build - but only in short games, after you play a while it is very useful to get him and buy its upgrades. They are that strong that it was better for balance to make them very expensive - thats why you don't see them that often. But to get back to the question if Gondor needs more benefits:
Gondor buildings with benefit:
farms
forge
normal house
stoneworker
statues (next Version you can build more of them, because they can also be build at the towers position)
Indeed the benefit goes much further than that if you look at the discounts they provide. Farms reduce cost of cavalry and their production can be increased through the Marketplace. Forge reduce cost of upgrades and idem. Town house reduce cost of Tower Guard, Guards of the Citadel and Dol Amroth Soldiers and idem. Stoneworker
reduces cost of building construction even if you only build one. Heroic statues reduce hero cost, but as we will see in the next version, this might not prove much stronger. Ask the question and do the math: risking your base to not be fully protected by arrow towers just for a hero discount will turn out more expensive.
What I mean to say is that there are many ways to get discounts, but particularly the Heroic Statue influences your strategy: either cheap heroes and strengthening units, or defending an empty camp against an ongoing assault with Towers. This synergy is very nicely build in by ET.
Rohan got 2 buildings with benefits, Isengart got 3. I don't think that Gondor is the next one in line that should get a new one. That would be a real advantage for Gondor.
We can also ask ourselves why we aren't complaining about Rohan's 'lack' of diverse resource buildings, of which one does not even give a discount on troops or upgrades. Isengard, on the other hand, also disposes of the Steelworks which give both the economy and standing upgraded armies a boost at once. A very ingenious option that reflects the thematic of Isengard's industry.
Here is my real point: Gondor lacks the
thematic of a new economy building. I disagree with the idea of a Iron Mine because they were not really known for mining, not as much as the Dwarves at least. You could say many of the non-native resources they needed came through trade, which is where the Marketplace comes in.
Gondor has only one type of economy building for settlements from all factions. From my perspective, I feel that second option would be beneficial. To benefit from farms (cavalry discount), you need to train horses even though you don't want to or need them in your current game. Let the fact of having no other options and strategies stay in vanilla. Edain is more interesting than that.
There are other strategies Gondor can play out with its settlements, if you are willing to risk it. Going all-in on Signal Fires gives you a large free army in online games if the opponent is not offensive enough. Besides, feeling obliged to spend money on cavalry because you build farms is not a reason: the benefit you would get of not spending money at all is greater indeed! Or, more to the point, Farms still produce resources and upgraded with a Marketplace I won't see you complaining if your settlements are filled with them and money is pouring in.
In short, I am not in favour of giving Gondor another Economic option, particularly on the settlements.