What kind of serialized story can you do with exploration?
Spend a whole season at each planet. A real exploratory mission wouldn't just drop in for a day or three and deal with one event and one place -- they'd settle in and spend months surveying the whole planet in detail. A full-season arc could let them visit multiple different nations and regions of the planet, get involved in developing local tensions and crises, have evolving relationships with people on the planet, etc.
But you need a story that builds up to something and you can't have an episode where they just do a survey, there has to be something exciting that. Unless there is a coup or something, what would the season finale be? Also if there is political turmoil on the planet, wouldn't the prime directive prevent them from getting involved, which means you have a season finale where the Federation just leaves when there is a coup, instead of helping to deal with it.
Plus why wouldn't the Federation have to defend from conquerors, space is a big place, its naive to expect everyone out there would be friendly.
Don't misunderstand me. There are plenty of aspiring conquerors and threats of war in Trek. But most of the time, war in Trek is a threat that is ultimately averted because the heroes are smart enough and committed enough to find a better way of resolving the conflict. (Or sometimes because glowing balls of light masquerading as medieval villagers force them to find a better way. Everyone has their off days.) Even the Xindi arc was ultimately a story about averting a war rather than waging one. Most of the time,
Star Trek is not about fighting in wars, but about fighting
against war. And that's a far greater and worthier challenge. Letting a war start is easy; stopping it or preventing it takes far more courage, strength, and skill.
But that's not always an option, the nations of Europe couldn't have just hoped that once the Nazis got into a power in Germany, a peaceful solution was not in the cards, despite Chamberlain willing to give Hitler the Sudetenland and pretty well everything else he asked in exchange for "Peace in our Times."
Sometimes it seems like the Federation was so adverse to war, that would make really bad peace deals with other powers, which the other side had no respect for. The treaty with the Cardassians was so flawed, it fell apart after a couple of years and the Cardassians did everything in their power to circumvent it when it was still in effect. That doesn't seem smart, it seems foolish. The Federation should seem more clever then get into bad treaties like that.
I just don't think an anomaly of the week show will be as a popular as it was in the 60s or 80s and is really played out in terms of what we have seen before in Star Trek.
So perhaps a Cold War between the federation and another power is the best compromise. Sure there have times where the Federation has been that state with another power, but how often has a cold war been explored in detail in Star Trek?
We never really saw the Federation have not to sway a non aligned planet to join them over another power and there could be an interesting story where this rival power wants something and the Federation races to get it first. Why does a non aligned planet only has the choices of staying non aligned and being part of the Federation, wouldn't joining the Federation mean more if another power is trying to convince that planet to join them instead? A long arc where the Federation and the other power are not at war, but the other power is trying to become the dominant force in the galaxy, by convicing non aligned worlds to join them and searching out resources that would make any future war a moot issue.