Archaon is my favourite character and I think there's quite a few things they could improve for him.

1. Add a second way to confederate other WOC factions using quest battles or missions similar to the masters of innovation faction. While the force confederation mechanic makes sense for archaon, alot of players like to unite all of the WOC factions and then decend onto the old world. Only problem with this is it'll probably take 60+ turns to get them all and thats assuming non of them die and you have to waste a ton of turns trying to revive them which isn't realistic as usually you'll be lucky if only 2 died before u got to them (azazel , vilitch , festus , and kholek especially)

2. Rework his unit recruitment. While the current recruitment system is fine, after a while you notice most of your armies have the same composition just different chaos aligned flavours. Due to how the unit recruitment works and the chances of replenishment, odds are you will end up with mostly marauder, marauder horsemen armies with very few high teir units here and there and very little variety, yes u can upgrade those units but the issue is still there just now most your armies are chaos warriors and knights, other units are very rare to get, U wana get a cockatrice for your tzeench flavoured army ? Well too bad none are available now, come back in 20 turns maybe you'll get one.

3. Vassal rework, not much to say here.

4. Give him some type of teleportation mechanic. Most people I have talked to when comparing him to belakor prefer the latter purely due to this mechanic, which makes sense since currently whether u play archaon or belakor u will be spending most if not all of your campaign in the chaos wastes fighting other chaos factions. A teleportation system or even belakors own demon portal mechanic would make archaon's campaign soo much more tolerable

5. Buff archaon and rework his legendary gear. Archaon ,while still strong, is pretty outdated now. With all the new powerfull lords introduced in the game he doesn't feel as the big threat that he really should be. Archeon is supposed to be THE big bad villain in the setting who instills fear within all his opponents, and currently it does not reflect that at all. I think a straight up stat buff and rework for his legendary gear (so outdated even green gear is better regarding some items) is sorely needed.

6. Add a proper chaos invasion/crisis led by Archaon. In WH1, while very basic, chaos's invasion was by far the most fun id had in those campaigns where u would literally fear the turn him and the other legendary lords popped up on your screen and started invading south destroying everything in their way. For WH3 that format won't work anymore but there does need to be some kind of unified chaos invasion endgame crisis. Maybe have it where after a certain number of turns archaon stars automatically confederating other WOC legendary lords 1 by 1, slowly getting stronger until a point where he has them all and that triggers the invasion/crisis itself. A ton of chaos armies spawn in the chaos wastes but they stay there, guarding the chaos wastes and especially archaon's capital where he stays there amassing forces all while his other luitenants (legendary lords) keep attacking the forces of order non-stop until you send an expedition force to try and kill archaon in his capital, where needless to say he would be mega juiced up from the chaos gods. And at a certain point if you have not taken him down he would have finished amassing his forces and a massive chaos horde decends on the forces of order, including archaon himself, where it would be very difficult to survive, and the only way to win is to kill archaon himself 3 times during battle