Its OP, I have seen the BM fighter around with a Rogue in the party and its a great trick, aciton surge for 3 attacks give 1 attack to the Rogue.
A dex based BM fighter (duelist) deals 1d8+7 damage (11.5 avg damage), they give one attack to the Rogue at level 8 for example 1d8+4d6+5 (23.5). Its a nice ability but its also not at will.
The Duelist style for example grants +2 damage, a level 8 cleric class feature grants a huge 1d8 (at level 8), thats an at will thing that doubles the damage (comapre with the crit thing of the 5E champion)
Replace the Rogue with a Ranger, 1d8+5+ 1d8+1d6 your are still getting an extra 8 points of damage out of it, and this is also missing out on things like the -5/+10 feats in an edition where the average AC in the MM is 14.5.
This also ignores the versatility of being able to switch the targets to use range/melee attacks and you can key it off the best stat of another PC (negating Clerics MAD downsides for example).
Its fairly close to the +10 damage part of the -5/+10 feats without the -5 part. Or Paladin smiting witout having to use level 1 spell slots. When you look at how much damage crits deal or class features deal the way at will attack granting is very very out of whack with other PHB abilities for dealing extra damage. Or how cantrips scale even.
If you compare it with extra damage and how it will be used in a real game of D&D its the best damage enabler in the game, its better than level 8 cleric abilities its better than a fighting style, its roughly equivalent to a Paladin using a level 1 spell slot to smite at will, its the situational advantage of the hunter ranger all the time. It worked in 4E because they had a basic attack and Rogue sneak attack only dealt an extra 2 dice of damage (and 4E critters had more HP).
If you allow it at all it should be dealing perhaps 1d8+3 damage roughly as that is roughly what a cleric or warlord would deal at level 1 and you still get the versatility of switching between ranged and melee or keying off the prime stat of another PC.
Clerics can situationally deal more damage using their spells of course, spiritual guardians comes to mind but once again that is a limited daily effect, I am only limiting WLs to short rest effects.
If its at will why not just have it grant an extra 1d8 (perhaps 1d10 be generous) that scales like a cantrip. You will more or less functionally have the same effect and it limits the brokenness of it. Similar concept (more damage), and it limits the abuse (like 4E did at least until essentials basic attack classes). It means you could have a lot more other abilites that you would associate with the WL.
It really does smack of one true wayisms, every other class got changed to fit the 5E design why should 4E get a free pass? Not that the loudest posters actually play 5E anyway. If you spent time designing a 4E retroclone instead of bleating about it on forums you could do whatever you liked with the WL.
Can you have a Warlord in 5E? Yes, can you have a fully functional 4E Warlord in 5E no for the same reason a fully functional 3.x CoDzilla is a stupid idea.