Looking down the 1st level at-will abilities, what have we got? Give an ally an off-turn basic melee attack, that's very 4e and only works because basic attacks are standardized.
That one is semi-true. But attack granting is still in 5e.
Command Flee to provoke an opportunity attack.
Haste to give an extra action.
Commander's stike.
Give an ally a numerical bonus on their next attack, that's the 4e numbers game there.
Numerical bonuses existed in 1e, 2e, and 3e.
But I doubt any Warlord fan would claim the class identity would be changed if it gave advantage instead of +Int to-hit.
Deny an enemy the ability to move without provoking an Opportunity Attack, that's 4e positional play at work.
So Sentinel is a 4e thing?
And give an ally the ability to shift 1 square, that's 4e off-turn actions and positional play again.
Litterally the dance bard 3 feature to move the ally 1d6 squares (5').
As a Warlord fan, would you really say that a 5e Warlord that has roughly a Battle Master Fighter's ability to do those things is really a Warlord? Of course not.
I have made a viable warlord by Frankensteining existing 5e mechanics together into a single class for a one shot. Not exactly elegant design, but it worked.
And that was several books ago. Stiff like Dance bard 3 would be a perfect addition to graft on.
A more elegant solution would probably be something like you get a d4 tactics die every turn, and a list of battlemaster / bardic inspiration stuff prepared at the start of each day.
At level 2, you get an aura with "when you roll initiative, roll your tactics die. Each friendly creature in the emanation (including you) can add the result to their initiative roll". The aura is there for subclass and choices later.
Level 3 is subclass.
- Cleric 1/3 caster with healing word and such.
- Fighter with extra attack.
- Ranger boosting one who is good at ambushes, giving a boost in stealth in your aura
- Psionic because that seems to be the 2024 thing, giving your party telepathy and if anyone can see the target, everyone can.
Level 5 you pick up a reaction.
Then you scale your tactics die, feats, more advanced maneuvers, add an effect to your aura, ect...