A very interesting and creative idea, but there are many problems I see in its implementation. First and foremost, if it's not a city, where does its production come from? Does it have a fixed spt output, or does it rely on the land around it? If the former, it will be tough to determine the spt to be useful yet not exploitive, plus this value will have to surely change many times throughout the game as unit costs change. If the latter, then how is it different from a city?
Second, what is required to build this outpost? If it is going to be something that can be built be workers, then it would be ridiculously over-powered. The whole point in having settlers and city growth is that investment is needed to make a viable production center. If I can get the same production of a city by building a few workers and spending a few turns to get some equivalent of a full production city, then why bother settling past a few cities? Worse yet, there is no population investment; workers don't go away when the outpost is built, allowing a team of workers to generate massive production, which is clearly unbalanced. I suppose one way to implement this would be to make the outpost like a colony; needs a worker to build. However, this has problems too, in that worker pumps are rather easy to setup, and there could be all kinds of exploitive use of this to spam production outposts all over the place and wildly unbalance the game.
However, the worst offender to game balance I see here is the negation of the supply chain effect. The biggest challenge to warfare in the Ancient and Middle Ages is the fact that the deeper you press into enemy territory, the further your army gets from its supply base. Your reinforcements take longer and longer to arrive, while your opponenet's time gets shorter. This has been a major factor in warfare since time began, and remains so even to this day (look at the vehicle armor problem the US is having in Iraq). It can even turn the tide of victory, as it has for the Russians time and time again as nobody has been able to overcome the long march to Moscow and successfully conquer the country. Being able to construct quick production sites would negate this, as all you need is a team of workers to follow the army, building outposts to provide quick reinforcements. The classic strategic choice, balancing the gain to be had by early conquest against the loss in growth due to building a large, slow moving army would be lost. The obvious strategy would be to build enough units to handle the first couple cities and a pack of workers, and BAM, you have a self-sustaining ancient age army that can march on anybody while your main cities continue to grow.
Anyhow, that's my view on the issue. I am NOT trying to attack you in any way; just posting my opposing view to your idea and the reasons behind my position. I love engaging in strategic debate, and everything said here is meant to be at that level, nothing personal
I would love to hear a more fleshed-out explanation of your idea, what you are envisioning, and why you want it there.
-dathon