According to the current state of the meta, herospam is, to put it bluntly, overpowered.
The main reason for it comes down to a simple fact, that is that the best way to fight heroes is with other heroes (because even the hero killer each faction have performs best if accompanied by other heroes).
To avoid having herospam as a far too good tactic, I suggest a new unit type is added to each faction (or rather, some currently underused units in each faction are "repurposed" and if ALL unit in a faction are already much used, new units are created, but I don't think that will be necessary).
That unit would play against heroes very much the same role the pikes play against cavalry. (and, just like the pikes are suboptimal when it comes to fighting against non cav units, these units would be suboptimal when fighting against standard units). This way you'd have a good old rock scissors paper including heroes as well, with heroes > standard units > anti-heroes units > heroes.
Now to the units that could play that role (probably with a tweaking in their cost and stats so they become stronger vs heroes and weaker vs units should that be useful), per faction. (some are currently empty, I'll fill them if people give ideas and enjoy the suggestion :
Imladris
The dunedain, that are currently barely used, would be perfect for that role imo. It makes sense lore wise, given they're descendant of great kinngs and numeronean , and they also are currently not really useful (at least, not for their troops).
Lothlorien
I'd say the ministrel is currently not used enough, but it wouldn't really make sense lore wise, and that would be a total repurpose.
Alternatively, maybe mirkwood units could be a solution, but they're already quite different from lothlorien unit imo, so I don't really know
Gondor
The dol amroth troops are as it stands rarely seen in gondor, so maybe giving them that purpose would make them actually useful. As for the lore accuracy, they're supposed to be elite troops, trained differently from gondor troops that are more used to a constant struggle against mordor.
(as for arnor, I'd guess the dunedain or high elves could be recruited from the elven fortress, or maybe if the heir to the throne hero decides to go as a ranger, the dunedain could get an upgrade making them valuables herokillers)
Rohan
Not yet any idea, maybe something from the archery range as it currently
isn't used at all imo doesn't have much use except for spearthrowers, but I don't really see what troops would play that role in a coherent way.
Dwarves
Not yet any idea, maybe lake-city / dale troops, but to be seen with the proposal for these outpost to be reworked what that would give. And the iron hills outpost units would undoubtly need to have a similar role as well.
Isengard
I heard the berserk weren't very used, so maybe making them deal only single target damage, but potentially heavy damages would allow them to play the herokiller role for isengard. (It IS also lore wise, given they killed haldir in movie).
Mordor
Mordor being mordor, I don't really know, they already have so much possibilities. I'd say the dol guldur outpost could produce these units instead of their free orcs, since it was in the hobbit movies the place sauron almost caught gandalf, so linking it to the anti hero units of mordor would make sense.
Angmar
Not yet any idea, I didn't play them even ONCE since release, but maybe zaphragor is already strong enough so they don't need these troops, but I don't really know.
And now, to get an easy view of the reception, the IN FAVOUR / AGAINST list that follows :
IN FAVOUR
Turin.turambar
AGAINST!josdos