So the wizard never gets caught with his pants down in the middle of a bad case of the runs brought on by that second-rate salted pork the party saved money on. The fighter never drifts back a bit while the cleric and the thief have another of their arguments. No one seeks their own space for their own thoughts - and of course, while camping, no one ever wanders more than ten feet from any other party member. Not even to get water or see to the horses.
I'm on record slamming that, arguing that the party should definitely be caught scattered in the woods, having to occasionally fight off the first attack by themselves, screaming in panic and hoping the rest of the party can get there in time. So please, let's take it that in my world, parties en route will drift apart, even if they don't like it.
However . . . suppose that one fighter in the party had a sage ability in Tactics. Suppose that as the party approached a particular bit of topography, a particular bend in the road, this fighter had a bell go off: "Hey, hey, everyone," the fighter calls. "Come here, come closer, I want to tell something."
So the party approaches. "This looks like a good place for an ambush," says the fighter. "Maybe for a mile or so, we ought to bunch up, stay close."
Do I have a problem with that? No, not at all. Even a mob can organize their thoughts well enough to stay focused for a mile, if the fighter saying so has already proven his good sense about these things. It is that last notation, however, that matters. What counts is that, when the party does bunch up, it turns out the fighter was right - it was a good place for an ambush.
So how would Tactics play out in a D&D campaign. Well, the above example for sure - reasonable expectation that the party was going to be ambushed. I'd like to propose a few others, all of them out of the ordinary and all of them adding to the fighter's ability without affecting attacks or damage:
- Time to Advise. If the reader's world is like mine, we both press the players to make decisions about what they're going to do in a crisis without feedback from others. Sure, before they march into the temple, I expect them to plan; but once things start happening, I insist they go it alone. Often, if one player gets a bit pronounced about telling other players what to do during combat, I take attacks away from that player. Obviously, they are using all their time talking to other people - they don't have time to attack. But we can always make an exception. A good tactically trained fighter could fight and be permitted to send a solid message to other players about what to do (which could be played out by using the other player's suggestions to enhance the fighter's actual tactical ability - no every player with this skill will necessarily know how to give a tactical suggestion).
- Sound Judgment. Most players won't take advice from the DM; in fact, I've run with a number of players who consider it anti-characterization . . . since, of course, their characters couldn't possibly know the DM's mind. But suppose, quite reasonably, that a tactically trained fighter could read the situation as well as the DM. That would be fair, no?
- Setting up an Ambush. Just as a player can see one coming, the same player ought to be able to plan their own. How? By setting the movement of the enemy so that at the moment of attack, every player in the party has a) the right range; b) initiative; c) a great chance to get their first missile shot in unseen (could be a wisdom check); and finally, we can adjust the enemy so that they're strung out in the stupidest way possible. Okay, yes, I know most DMs do that for the party anyway . . . but now there would be an excuse!
- Cover and Concealment. In wilderness travel, a greater chance to move closer and surprise an enemy in their camp or lair, as well as a greater chance to avoid patrols, both in the wilderness and in an urban environment. In effect, a chance to read the situation and determine the best possible approach and the best possible hiding places. In rules terms, this could mean slight modifiers for a dozen or so things - which in composite, would make a big difference, especially in large parties. A slight adjustment is probably best, otherwise it might over balance the system.
These are just a few ideas. The bigger picture is in the DM adjusting the situation to make things easier for the party; ensuring that the place where the party goes over the wall has a minimum of guards (because naturally the tactical-experienced character 'chooses' the right wall to climb over). The party could effectively dodge patrols until finding one that seems to possess guards or soldiers likely to listen to a parley. A trap placed has a greater chance of someone walking into it, because the tactical fighter knows just where someone will step while walking along a trail. If there are two groups of missile-users, the tactical fighter's decision to split them up will result in a crossfire - because the enemy will be caused by the DM to walk right into one. This sort of thing.
A bit too much power for player characters? There are ways to get around it, the most obvious would be to increase the number of enemies or give them a tactical fighter of their own. And a chance, always, that the tactical fighter does make a mistake. This is always a possibility!