That is, I can consider suggestions to change the abilities of certain units or even make simple visual changes, such as recoloring, adding special effects, or changing weapons. But suggesting to completely remove certain units and replace them with others is too much, it's too late for such radical suggestions.
Well, I'm essentially suggesting a recolor. Leave all abilities and stats in place, but replace the dragons with messengers, and the messengers with knights. There's no need to repaint anything – just swap them. If zombies are given the abilities of other units, what's the problem with giving messengers the abilities of dragons, and knights the abilities of messengers? Although there's already a mod for knights – just add it and update the stats. And let dragons guard various crypts and dungeons – that's where they belong.
Added 6 minutes later
Messengers (damage dealers) and dragons (debuffers-tanks) already work well as a "team"; knights can conceptually only replace messengers (I don't see them in the role of debuffers-tanks, as that contradicts their essence). But Death with a scythe looks no less impressive (it's a matter of taste) and is more original than a dead copy of a human knight. Their problem in Heroes 5 was weak perks and tactical insignificance compared to death knights, which was fixed.
Try playing a few games as undead in Heroes of Might and Magic duels and get the hang of it – then these questions will disappear, and my objections will become clear.
I'm not against messengers. Messengers and knights are great – much better than the overused bone lizards. It's a breath of fresh air. I'm not suggesting changing skills and stats at different levels. Just replace messengers with knights, and send messengers to take the place of dragons. Give the current abilities and stats of messengers to knights, and the abilities of dragons to messengers. Tactically, nothing will change – just a change in appearance. And if it's completely impossible to give messengers flight, then you can add a couple of squares to their movement range as compensation. And I have played as undead – it's great. But the absence of dark paladins is still disappointing. They were one of the most memorable units.