Perhaps while talking about whether or not it'd be a good idea to include petty kingdoms, it'd be a good idea to discuss which titles the "Petty Kingdom" title is meant to represent? As a result, I compiled a list of all petty kingdoms in CK2's bookmarks and looked into which historical titles they were meant to represent. Being well-informed about the actual nations which the game labels as "petty kingdoms" might be a good way to provide solutions for the issue at hand.
In CK2, a petty kingdom is a simply an independent duchy whose ruler has a culture which enables petty kingdoms, these cultures and culture groups being:
* North Germanic (Norse, Swedish, Norwegian & Danish)
* Anglo-Saxon
* Celtic (Irish, Scottish, Pictish, Welsh & Breton)
* Sogdian, Tocharian, Saka
* East African (Ethiopian, Somali, Nubian & Daju)
* Nepali
* Tibeto-Burman (Bodpa, Tangut, Zhangzhung, Sumpa)
NORTH GERMANIC
In bookmarks from 1066 onwards, there are only two North Germanic petty kingdoms (technically three, if you count Erik the Heathen's revolt in 1066 as a kingdom): Iceland (which is a misnomer, since Iceland was never actually a kingdom, but rather a commonwealth), and Kingdom of the Isles, which could simply be represented as a kingdom-tier title instead (as HIP and a bunch of other mods do). In the bookmarks before 1066, however, there are plenty of Norse petty kingdoms.
Since the history of Norse political entities prior to the formation of Norway, Sweden and Denmark is spotty at best, I'm not trying to map out which historical kingdoms each petty kingdom match. Instead, I'll just list the petty kingdoms and name their in-game rulers.
In 936 bookmark, there are six Norse petty kingdoms in addition to Iceland and the Isles: the Swedish petty kingdoms of Sviþjod (ruled by
Ring of Sweden), Hälsingland (ruled by Borkvard Ulvo), Småland (ruled by Frodi Wulfing) and Vestergautland (ruled by
Skoglar Toste); the petty kingdom of Nordreyjar (AKA Northern Isles, ruled by
Torf-Einarr), and Namsborg (ruled by Hakon of Namsborg), represenging Viking occupied Nantes.
In 867 bookmark, there are a total of ten Norse petty kingdoms, most of which appear to me to exist solely to allow legendary or famous Norse leaders to have the power-base they need to do their historic antics, and either are the nucleuses for historical kingdoms like Norway or Kievan Rus, or were minor historical states. Four of the petty kingdoms are ruled by Ragnarssons: Sjaelland (ruled by
Sigurdr 'Snake-in-the-Eye'), Sviþjod (
Bjorn 'Ironside' af Munso), Jorvik (ruled by
Halfdan 'Whiteskirt') and The Isles (ruled by
Ivar 'the Boneless'). Five more are ruled by famous or legendary Norse rulers of whom I was able find sources for: Jylland (ruled by
Bagsecg), Viken (ruled by
Haraldr 'Fairhair'), Vestergautland (ruled by
Hrane the Geat), Holmgardr (ruled by
Rurik), and Könugardr (representing the principality of Kiev, ruled by
Dyre 'the Stranger', also known as Askold). Finally, there's one more petty kingdom of whose ruler I was unable to find sources, which could imply that the kingdom might be fictional: Smáland (ruled by Vagn Njudunge).
And in 769 bookmark, only 4 Norse petty kingdoms exist: Sjaelland (ruled by
Harald Wartooth), Sviþjod (ruled by
Sigurd Ring), Austergautland (ruled by
Hjörvardr Ulfing), and Vestergautland (Oddr Gautske [supposed ancestor of Hrane the Geat, of whom I couldn't find easily-accessible sources]).
ANGLO-SAXON
There are only seven Anglo-Saxon petty kingdoms in 769 and 867 bookmarks, which represent the Heptarchy (apart from Sussex) and Hwicce. However, since 769 bookmark is not going to be in CK3, we don't have to worry about representing the Heptarchy in its prime. CK3 will have 867 bookmark, however, which features the last four surviving Anglo-Saxon kingdoms: Wessex, Mercia, East Anglia, and Northumbria.
As a result, it might not be out of question to simply represent the four Anglo-Saxon kingdoms as kingdom-tier titles. "HIP - Historical Immersion Project" mod does so by having all four Anglo-Saxon kingdoms be de jure titles in 867 bookmark, but once the Kingdom of England is formed for the first time, all four Anglo-Saxon kingdoms will immediately de jure shift to England (apart form the northernmost part of Northumbria, which becomes de jure part of Scotland instead). Personally, I'd like to see this approach being taken in CK3, as it'd make the formation of England feel more impactful.
CELTIC
The minor Celtic kingdoms on the British Isles and in Brittany are basically the text-book definition of a petty kingdom: independent states which were described as kingdoms at the time of their existence, but were significantly smaller than later, more centralized kingdoms. Given that many of these petty kingdoms exist across multiple start dates, I'll just list them here and provide links to the kingdoms they're meant to represent.
- Irish: Mumu (representing the Kingdom of Munster), Connachta (representing the Kingdom of Connacht), Mide (representing the Kingdom of Meath), and Dal Riata (representing the Gaelic kingdom of Dál Riata)
- Cumbric: Strathclyde (representing the Kingdom of Strathclyde)
- Manx: Manaw (representing the Isle of Man under the rule of Elidyr, the father of Gwriad Manaw, the mysterious founder of the Merfynion dynasty)
- Cornish: Cornwall (representing the Kingdom of Cornwall/Dumdonia prior to the English conquest of Cornwall)
- Welsh: Gwynedd (representing the Kingdom of Gwynedd), Powys (representing the Kingdom of Powys), Glywysing (representing the kingdom of Glywysing), Morgannwg (representing the kingdom of Morgannwg, the successor-state of Glywysing), Seisyllwg (representing the petty kingdom of Seisyllwg), Deheubarth (represents the kingdom of Deheubarth, the successor-state of Seisyllwg)
- Breton: Brittany (representing the Kingdom of Brittany, although Breton-ruled dukes of Brittany are still listed as petty kings)
SOGDIAN, TOCHARIAN & SAKA
There are only two independent Sakan petty kingdoms: Hvamna and Kashgar, representing the Kingdom of Khotan and the Shule Kingdom, respectively: presumably Tocharians have petty kingdoms in order to give a more accurate name for the Tocharian oasis states in Tarim Basin (which by 769 had lost their independence). In addition, there's a Sogdian petty kingdom in 769 bookmark, which represents the domains of Abbasid official Salim of Farghana.
EAST AFRICAN
There are a total of 11 different petty kingdoms in East Africa, but since many of them stay the same across bookmarks and I don't have much to say about them, I'll quickly summarise which states they are meant to represent.
- Nubian petty kingdoms represent the kingdoms of Nobatia, Makuria and Alodia. These could potentially be represented as de jure kingdoms instead: after all, an unified kingdom of Nubia never existed.
- I was unable to find any records of a Somali petty kingdom called "Berbera", so I presume that the title is just used to represent the Somali clans that controlled the area.
- The Ethiopian petty kingdoms represent historical kingdoms which frequently were vassals to the Ethiopian Empire: Gojjam, Damot, and Gondar (also known as Begemder). Solomonic petty kingdom of Axum probably represents the rump Axumite state.
- The petty kingdom of Shewa appears to represent the Sultanate of Showa, while the Jewish petty kingdom of Semien/Axum represents the Kingdom of Semien.
- The Daju petty kingdom of Wadai appears to represent the Daju kingdom.
NEPALI
There were not a single petty Nepali kingdom in the bookmarks I checked, so I cannot verify which historical nations the title was meant to be used to represent.
TIBETO-BURMAN
There are a total of seven Tibeto-Burman petty kingdoms available across different bookmarks, of which I was able to find historical matches for only four: Maryul (representing the
Maryul of Ngari), Sumparu, Nagchu, Qampo, Dege (representing the
Kingdom of Derge), Minyak (representing
Dingnan Jiedushi, the predecessor of Western Xia) and Yartse (representing
Khasa Kingdom).