Here's a rough idea for it as a PrC.
You get a pool of dice that refreshes every round. Unused dice do not roll over. Starts at, say 1, and goes up to 5. The dice start as d4s and grow one step each level, ending at d12s.
Sounds like it has potential, though also the potential to be dangerous. Also: only five levels? Interesting. Usually people suggest it as a through-to-20 option (e.g. if it requires 5th level stuff, it's a 15 level PrC), in part because a 5-level PrC means giving up any chance at "capstone" abilities.
Every level, choose 1 battle tactic from following list. Max 5.
Hmm. On the one hand, I understand using "limited access to tactics" to prevent excesses. On the other hand, five and
only five tactics isn't a lot. I haven't yet looked at all the tactics, but I worry that this might be a little overly limited. Possibly a more interesting option: pick two tactics at 1, two at 2, two at 3. At 4 and 5, pick a tactic (or two) to enhance to a superior form. I'll give an example later.
Spend N dice to allow N allies to move half their speed without provoking opportunity attacks. Uses your action.
Decent. Covers the whole "reposition!"/"flank 'em!" side of things.
Spend one die to grant an ally an attack. (Uses your bonus action OR their reaction, adds value on die to attack roll or damage roll).
Also sounds good. This is where that "advanced form" comes in: initially you can only grant the attack by expending the target's reaction, but the "advanced" form lets you choose either your bonus action or the target's reaction. Optional limiter, if people are still concerned about the Sneak Attack issue: "a particular ally cannot benefit from this feature more than once per round."
Spend one die to grant temp HP (uses your bonus action, adds your cha to the amount granted).
At early levels, this is good--but at later levels this becomes pretty damn weak. Even with a maximum roll and maximum Cha, you're talking 17 THP--average of 11.5. I suppose you refresh it ever round, but if it costs you your (bonus) action to do so (and thus you can only THP one person a round), you're still talking about losing HP even to CR 3 and 4 threats--and CR 10+ will blow through it like it's not even there.
Spend N dice to reduce damage from an attack by amount rolled plus INT modifier. (Uses your reaction).
Spend 1 die to grant advantage on a save (reaction).
Good for a 'basic' maneuver, but runs into the issue of failing to stack with the (many) ways one can acquire advantage on saves, especially vs. magic. If you did do the "advanced" versions, I'd suggest "advantage on the save, and the target may ignore any one source of disadvantage." That way,
Spend 3 dice to raise up an ally that has fallen to zero HP. Uses your action. Stands them up and grants temp HP equal to the amount rolled for one round only (or whatever). Ally cannot benefit from this again until after a long rest.
Seems like a lot of limitations stacked together here. Once a day per person (fine, often suggested for this sort of thing). "Stands them up" is left poorly-defined. Do they regain 1 HP, or are they simply ignoring the effect of being unconscious for one round? And the THP only last a single round? Sounds like the Warlord is pretty crap-tastic when the feces hit the whirling device--a Warlord brings you back up and you best be running away, because if you don't you'll go down again and the Warlord can't do diddly about it.
Spend 5 dice to restore HP to an ally by amount rolled. Uses action. Cannot use again until you take a short rest.
Interesting. That's...a pretty significant amount of healing at the highest values. Average of 5d12 = 32.5 HP. At the very highest levels it'll start to wane in effectiveness, but a Warlord who dedicates all his actions to healing (once people have taken damage) would be seriously competitive with Life Clerics, which seems odd--and a tad excessive--to me.
Spend N dice to increase ally's movement speed by amount rolled. (Uses your reaction).
Would need a duration, but otherwise sound.
Spend 1 die to exclude an ally from a friendly fire area of effect (uses their reaction).
I assume this can be done on the friendly-firing person's turn, rather than the Warlord's or victim's turns? Needs some more rigorous explanation before I can really respond.
Spend N dice to counter "surprise" and allow N allies to act. Use when rolling initiative - uses your next action, must include yourself.
Use N dice to replenish an allies HD. Ally can only benefit once per long rest.
I like this one. Hit Dice are currently pretty sharply limited, and there aren't too many things that interface with them (Song of Rest is probably the only one, and it's pretty minimal). Helping people draw out their internal reserves sounds good to me. Would need some testing to make sure it's not overboard, but it's a good idea.
Expanding on my earlier comment: I feel like it would be more interesting to give (say) a total of 10 "tactics" over the course of the PrC. First, because that means you have at least two "options" even to start off. Second, because you can do as suggested above, and have the last 2-4 slots be "advanced" tactics, which take a lower-level tactic and make it even better. Reduce limitations, or increase effect, or enable extra (though still limited) uses, etc. Or, instead of taking the advanced options, the PrC Warlord can get more of the basic ones--going for breadth over focus.
Some other options: the "not on my watch!" idea (enemy hits an ally, you spend dice to force a reroll or cause it to fail if the rolled value is higher or...something), boosting AC or saves, increasing an ally's save DC for their offensive spells, allowing one or more allies to act as though they were proficient in skills they aren't (possibly with Expertise or Advantage if they are already Proficient). None of these are even remotely attempted to be "balanced" yet, just suggestions for possible tactics options to increase the breadth of options available.
You also might want to consider what "fluffy"/"ribbon" features the PrC provides, since the Rune Scribe has several (some locked up within the runes themselves).
