Off-the-cuff attempt, based on Mearl's framework.
I think, conceptually, I would veer towards the types of generals in the "Kingdom" manga — either instinctual, or strategic. The Strategic generals work off of careful planning, while the Instinctual ones react to the 'feel' of the battlefield on the fly. Given that the strategic types tend to lead from the rear, while the instinctual types tend to lead from the front, as a character class the instinctual types would be the easier match for a PC.
Types of features/abilities:
Battlefield Awareness. A concentration effect.
While Battlefield Awareness is active, various abilities may be available (gain over levels?):
• Increases passive perception while active (easier to spot enemies trying to hide or sneak up on you).
• Hidden creatures cannot get advantage on any attack against any ally that can hear you. (range?)
• You may use a reaction to grant allies advantage on a saving throw against an area effect spell or attack.
• When an ally moves, you may use a reaction to prevent opportunity attacks against him.
• When an ally retreats behind another ally, you may use a reaction to command the second ally to assist the fleeing one. Enemies may not pursue past the intervening ally during this round. (battlefield control element; makes Disengage more valuable when retreating)
• You recognize a section of the battlefield as critically important, and a key point for events to happen. You designate this area at the start of combat. (size?) On activation (using a bonus action), until the end of your next turn, all allies within that region have advantage on their attacks, and enemies in that region have disadvantage on attacks against your allies and saving throws. Causes one level of exhaustion when it expires.
Command: You may expend an Action Surge in order to allow an ally to immediately take an action as a reaction. (Essentially, giving Action Surge to another player)
Charge: [Folded this into Command. An Action Surge can be used to Dash, so if you want to give someone a free move, that doesn't require a different ability.]
Formation: As an action, you may call for the repositioning of all allies within range. All allies may immediately move 10' towards a prepared formation of your design. Opportunities Attacks have disadvantage against those who move.
Draw In: You may use a reaction on your turn and designate yourself or an ally. Enemy units have their attention drawn to the designated individual. Those that are not currently engaged will treat the designated individual as their primary target, barring other direct commands, and move towards that person at the next available opportunity.
Prescience (stolen from mellored, renamed): When an ally takes an action that fails, including casting a spell, attack, or ability check, you can expend your action surge to warn them of their impending failure. That action did not happen, and they can take a different action instead. You can use this feature after you see the results of a roll, but before it takes effect.
This setup uses reactions as a major resource, which is not in conflict with a fighter's standard design. It also uses Action Surge as a resource, so the player is more likely to hold onto it, rather than spend it immediately, as most fighters would. It focuses on giving you advantages, and preventing the enemy from getting advantages.
I avoided others of mellored's suggestions, as the tactician's role is to put people in the best position to do what they do best, not to "make them better", which bleeds into buffing (such as the example of Bless that Mike Mearls referenced). That means helping people move (either granting a bonus Dash action, preventing Opportunity Attacks, or allowing other allies to interfere with pursuit of a retreat), and preventing the enemy from gaining advantage from its own positioning.
However that's leaning more strongly towards defensive features, and we want some offense, too. Command gives an ally the ability to "strike while the iron's hot", so to speak, giving another person the ability to act in your place, when they're better positioned to do so. And the "key battlefield zone" gives you a chance to nova with the entire party. (I'm not sure whether it's over- or under-powered, though.) Formation and Draw In can be used as a combo to pull the enemy into your key battlefield zone.
The Battlemaster does have access to some of these features, but the maneuvers available to them are more tightly bound to individuals and individual fights, while the Warlord should be attempting to work on the entire party. This take on the idea focuses on maneuverability, and then being able to nova off of good repositioning tactics.
Anyway, I didn't try to fit this into a leveling structure. It may be too much for a subclass, particularly for a class where it comes in at 3rd level. Taking a quick once-over, it'd be something like:
3: Battlefield Awareness (improving over levels), proficiency in Insight (tactics)
7: Command
10: Formation, Draw In
18: Prescience
Eh.. it's a rough sketch. Since most of it is folded into Battlefield Awareness, it might not even really be a problem, leveling-wise.