El Mahdi
Muad'Dib of the Anauroch
5e is still exception-based design, so some exceptions are going to be inevitable, though maybe that's not exactly what you meant by 'exceptions.'
I meant something like if one made a mechanic for the Warlord class that allowed them to restore a character at 0 hit points by using temporary hit points, for it to seamlessly drop in for an official class it would likely require adding an exception to the text of the basic rules for clarity purposes. Like: If you have 0 hit points, receiving temporary hit points doesn't restore you to consciousness or stabilize you. (exception: the Warlord's Inspiration ability)
Essentially anything that requires altering the text or rules of the basic game.
P.S.: the temporary hit points idea above is purely hypothetical and not an idea I'm pursuing.
Another possibility is making some things, especially 'tactical exploits' say, more situational than usual for D&D abilities (even compared to spells, which get highly situational), and correspondingly more numerous/available/powerful.
The problem I see with this is it sets up a need for both player and DM to remember or list a bunch of different situations or triggers that may be exploited. DM's so they can plan challenging encounters with a Warlord character in the group, and for players so they don't miss an opportunity and get disappointed/frustrated when they do.
I don't want a Warlord to slow the game down, but do what they do while keeping the game moving as clean and fast as it already does.
At the risk of seeming as pessimistic and cynical as I actually am, Mike Mearls has publicly expressed disdain for the very concept of the class.
This doesn't seem pessimistic or cynical to me, just inaccurate. Mearls has not expressed disdain for the Warlord. However, I don't deny that many have chosen to interpret what he has said in this manner.
Fascinating idea, but how would you get something that meshes well with both the Fighter's multi-attacking and casters' spell progressions?
The multiclassing rules already take care of that; and I'll provide a section within the class outlining its specific multiclassing guidelines (ones in-line with the multiclassing rules).