What's needed is an unrestricted ability, but one that is restricted by its own subsystem (such as spell slots, superiority dice or sorcery points)
Remember that the action economy degradation doesn't need to be only paid by the Warlord. I think the canonical example of this is the Battlemaster's Commander's Strike.
Battlemaster uses on attack (which may be only 1/2 or 1/3 of an action) to grant someone else an attack (which might be a full action for them and/or benefit from being on a different turn like the rogue). It also gives a boost to damage. The costs are the reaction from the receiver, which means that 4 warlords couldn't trigger the same rogue one after another, and it's using a limited-but-not-too-limited resource from the Battlemaster, a superiority die.
Now, from a design consideration, "pay now for someone to get something later" might also be considered a degradation. An example of that would be granting the equivalent of the Haste action. I think that's a valid concept to explore as well.
And the Commander's Strike isn't good enough for a proper Warlord.

It's restrected, because it's given to an honest-to-gawd fully kitted
Fighter.
As in, it has its place in any Warlord-y character's toolbox, sure, but for the "real" Warlord, we expect more - we expect at least a few
unrestricted action trades per rest.
For a fighter, it's much less desirable since, well, you're a Fighter - take you goddam swing yourself

The biggest cost is the opportunity cost of not doing what a Fighter does - that is murderizing BBEGs like no other character can.
Meaning, one of the core design parameters of the Warlord needs to be that its allies should be better at doing stuff than the Warlord itself

- that's why you would want to give away your actions in the first place!
As a corollary of this - you can't base the Warlord on a best-in-class class chassi (like fighter, rogue, cleric or wizard). You want the Warlord to be its own, decidedly mediocre, beast (except for its ability to "be" those other classes and "borrow" their top-notch abilities.
The closest you get, is probably Cleric. Assuming there was a completely non-religious, unmagical, Cleric, that is...
