One way to thread this particular needle is to give all warlords medium or light armor and make heavy armor something that particular subclasses get.
I think the overall warlord class has a bit of an issue with identity (in that fighters and bards are both very good narrative substitutes), but it's no worse than the psion or the artificer. You can make a distinction.
There is a
bit of a deeper issue with warlord subclasses in that the 4e versions were all "here is
how I do my job," and 5e's design methodology is that a subclass is instead a particular narrative archetype within the class's aesthetic. Basically. "I warlord tactically!" vs. "I warlord INSPIRATIONALLY!" is not a very strong distinction. These aren't really different characters. A leader-y martial buff class would do both. It's not like we divide bards up based on if they do stringed instruments or percussion.
But, riffing off of the point above, I do think some thought out subclasses could help that, and you can still use 4e's versions as a base to riff from, you just need to hang a bit more archetypal weight on each variety.
Riffing on that idea for the "initial 4 subclasses," I could imagine something like:
- The herald subclass uses shouts and cries as their bonus action, and is pretty good at empowering allies in the attack. This leader gives buffs pretty directly.
- The tactician subclass has bonus actions related to actively gaining buffs - the tactician uses a bonus action on their turn to let someone do X, if they do X, they get a bonus. This is the leader that hands out attacks and movement.
- The banneret might create auras as a bonus action that buffs creatures within them. Give them heavy armor and a shield so they can protect those in their radius.
- The bravo subclass gets a big ol' weapon and applies conditions on enemies, with stunning and disabling blows.
If you get martial weapons and light armor to start with, then these subclasses might grant armor proficiency, shield proficiency, possibly weapon masteries (like for the bravo and tactician)...
IDK, those are still pretty similar, but maybe we see the start of some differentiation...