Evil Stepmother. In a 3.5 Splat that would have been worth at least a 10 level prestige class.
Sure, monsters got classes in 3.5, and a monster- or antagonist-NPC oriented PrC would've been perfectly reasonable.
In 5e, as in the classic game & 4e, PC classes are their own thing, and monsters are their own, different things.
I can definitely see room for another support class in D&D. But how important is it that it retain any of the warlord's flavor?
Very. The class could be broadened, but the 6 or 8 various 'builds' (4e equivalent of sub-classes in 5e) possible with it should all still be made available.
Most support classes outside of D&D seem to be magical in nature.
You mean archetypes or tropes as 'class' in that sense is a D&D convention. And, no, outside of D&D, supporting characters are very often non-magical, as evidenced by the fact they exist in genres that do not include magic, at all.
OTOH,
in D&D, support classes have been overwhelmingly magic-using, generally full casters. The Cleric was the first, iconic, D&D support class, relying exclusively on its spells for that function. Since then, Druids, Bards, Paladins, Rangers, Favored Souls, Artificers, Shamans, and Ardents have all provided magical (or, in the case of the Ardent, supernatural, if not technically magical) sources of support. Magic is profoundly over-represented in the role.
In that sense, the Warlord is badly needed.
