Shouldn't the Imperial and Chaos warships be identical at this point in time?
For fun I am gonna speculate on what might be happening there. God knows I love starships.
In Battlefleet Gothic, they explained that Chaos had got their distinct starships from an Imperial shipyard, but their keels were laid down under a different (later) Imperial era than the standard ships of the Gothic Sector's navy. However I think that they probably represented a parralel human design lineage from the Dark Age of Technology, equal in antiquity to the familiar designs employed by the majority of Battlefleet Gothic. Charitably, we are probably expected to believe that by chance, the two lineages have ended up on different sides again in that artwork, even back during the Horus Heresy, like a legion wearing Mark III 'Iron' pattern Astartes armour vs. one wearing Mark VI 'Corvus' pattern Astartes armour. Yep; it's very unlikely, and they shouldn't keep doing this.
In favour of that interpretation, the Terminus Est, dating from the Horus Heresy, is already constructed in the 'Chaos style', and the Imperium employs an aged Grand Cruiser design in the Gothic Sector which shares aspects of it's overall appearance with the warships of Chaos (Grand Cruisers as a separate tonnage having fallen out of favour with the Imperial Navy, but some antiques remain in service).
What is probably happening is that the human starship designs of the 41st Milennium are meant to be quite diverse, perhaps representing different ideas fielded by long-dead planetary governments, regional empires, or galactic federations, from before the Age of Strife. I would guess that all of the above are valid designs seen regularily within the Imperium, including the typical Chaos one. Some might share the same superstructure/class, and only differ in their outward appearance, or prow (like the Adeptus Mechanicus variant of the Lunar-class). In a real navy, there are a lot of ships that are of the same 'class', sharing the same superstructure, but look quite different, having different turrets or towers on deck (unlike say planes, which are identical in a given production).
An extreme example of this might be how Yamato's sister ship Shinano was converted into an aircraft carrier:
One of course couldn't convert say, an F35 Lightning II into a Flying Boat easily, but ships are more flexible.
Below is an example of two Dictator-class battlecruisers, laid down perhaps at different Forge Worlds (Mars and ????). The Imperial Navy obtains ships from wider sources than the machine cult's Forge Worlds, including shipyards above Fortress Worlds, Civilized Worlds and Hive Worlds. Perhaps they additionally have the Tech Priests build their vessels to a certain preferred design pattern, and don't like the one employed by the machine cult's own explorator fleets:
The first Dictator-class is obviously employed by the Adeptus Mechanicus, while the second is obviously employed by the Imperial Navy. They are the same class, with roughly the same armament of macro-cannons and launch bays, but the Imperial Navy favours a naval ram. What might be going on there, is that the Dictator-class superstructure was a shared ancient design across many different planets (perhaps a universally known STC pattern), before the Age of Strife, but the different cultures, isolated by warp storms, took the lineage in different directions. Or perhaps, the design split is even more ancient, and had already been developed in different directions by long-forgotton political entities during the Dark Age of Technology. On the other hand, it could be modern, with the Imperial Navy and Adeptus Mechanicus having both customised an ancient frame to their liking. Were these designs once employed as science vessels, colony ships, or bulk freighters, aeons ago?
There are some good fanon designs:
Something that might be interesting to know is what the fleets of the Horus Heresy era looked like, when the Imperium's forces operated in combined legions; what proportions of major Expeditionary Fleets were made of what classes. Did some legions favour certain design lineages (perhaps having a specific Forge World assigned to supply them)? For example "Iron Warriors use X, Word Bearers use Y, Ultramarines use Z". We know from the texts that there were Space Marine battle barges and strike cruisers in the fleets. Probably mixed in with Imperial Navy ships of the usual classes. Maybe Mechanicus ones too. So maybe specific legions naval arm used either Gothic Sector style Imperial or Chaos style ships depending on their Forge World of origin. The Gloriana-class, given to each primarch, seem quite different from one another in outward appearance, and don't follow Space Marine tropes.
'Macragge's Honour' (pictured above) looks very different to say the Iron Warriors 'Iron Blood' (pictured below):
Neither ship looks like a later stereotypical Space Marine battle barge, but more like an Imperial Navy battleship.
A notable Gloriana-class that differs from both is Horus's flagship, Vengeful Spirit, which has two different appearances:
Possibly she underwent radical refit, stripping her superstructure, over the 10,000 years, at shipyards in the Eye of Terror.