I know it might be quite boring to other people, but I would honestly like to see a series set in a Federation Colony. I would set it post-Dominion War, in a Federation Colony on the Cardassian border.
Background: The colony is located in the Demilitarized Zone, but remained in Federation control, so the citizens of the colony - although sympathetic to the fellow colonists who found themselves under Cardassian - were never explicitly involved in the Maquis. Nevertheless, the colony (which I like to call Nea Salamina - 'New Salamis') received a large amount of refugees during the war; refugees fleeing the Cardassians and the Jem Hadar. Now, many (though not all) of the refugees want to return home, yet Starfleet is unwilling to allow the refugees to return, as they do not want to be seen to be taking advantage of the destruction of the Cardassians, especially not by retaking old colonies.
Tensions arise as the refugees become disillusioned with Starfleet, and feel abandoned by the Federation. Whereas the citizens of Nea Salamina, feel like their hospitality has a limit, and are unwilling to provide permanent homes, jobs, etc. This would give writers the opportunity to discuss 'real world issues,' as they are fond of doing.
Characters: The series would follow different colonists, refugees, and Starfleet personnel, as they attempt to live their lives on the frontier of Federation space. As it is a Federation colony, it is open to all races of the Federation. Its location on the edge of the Demilitarized Zone makes it of strategic importance to Starfleet, as its their last outpost before Cardassian space (so it is in their interest to keep the colony on their side, and stable). The colony might even have some Cardassians seeking to make a new home after their defeat in the Dominion War.
Some Plot Ideas: This could allow the exploration of the Federation's views on inclusiveness and equality, etc., as having Cardassians in the colony would lead to interesting relations between the Cardassians, the colonists, and the Federation refugees. Maybe the new Cardassian government has a special eye on the colony to see how their citizens are treated in the Federation, and whether the Federation is serious about establishing better relations, and providing assistance with rebuilding.
I think it would also give writers the opportunity to explore other facets of the Star Trek universe e.g., the relations between Starfleet and civilian governments; how things like trade, and providing for your citizens' needs, works on a Federation outpost; Federation arts and culture; maybe it could answer the questions we all have on how society functions in the Federation; the increase in population could allow for the further exploration, and surveying, of the planet, or maybe its location makes it a popular place for Starfleet to meet non-Federation members, in a location that is not Deep Space 9. Admittedly, the opportunity to seek out new life, and boldly go where no one has gone before is bit limited by such a concept.
Finally, I've always had this idea for an episode, where the colonists are excavating in order to expand the colony (or whatever), and they come across some mysterious objects. The archaeologists/historians are called in and they discover a proto-Cardassian, or proto-Bajoran, civilization. The Federation archaeologists explore further and find skeletons, graves, works of literature etc. But the Cardassians, or Bajorans, discover the Federation's findings. This leads increased tensions with the Cardassians, or Bajorans. If their Cardassian, maybe they use this an excuse to support the superiority of their people, or to demand the Federation hands over the colony to them, as they clearly colonized it first. If their Bajoran, maybe they demand the Federation ceases the desecration of their ancestors graves, and religious sites, or maybe they too ask for its return, to its 'rightful heirs.'
Thanks for the opportunity to say what I would like to see in a show, and thank you for reading!