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Realm stances and conditions (incl. some very few scripted events) at the start
1) All pagan/heathen realms would be somehow friendly, quasi allied, aka do not attack each other too early on ie. until year 871 AD or so, but maybe the game-mechanics will force that anyway. They would also have quite some startmoney due to yearslong raiding and better/bigger armies and especially more capable ships at the start.
1a) Danish/Norse raiders, small raids with capable units, they are okay already in vanilla imo., no change.
2) Irish realms are very poor and rather fight each other and get attacked by Norwegian Viking realms. While their warriors (medium to higher tier units) are pretty capable and very high morale, but also more expensive in comparison (here could be the only necessary unit modding impact, as i guess the other faction units are quite okay balancing-wise as is).
3) Same as 2) for the Pictish/Scotish realms.
4) Welsh/Cornish the same as 2) + 3). While at least for the Welsh and Pictish, one should avoid distinction, so ie. their maincenters should be very strong. The british factions ideally would fight for existence, just do not expand, but also do not disappear from the game (as in VC vanilla observable by me for the Welsh factions, annoying imo.)
4a) And the Irish perhaps likely ally some day to hunt the Norse invaders from the isle, some of the latter which press then as well into Mercia and Northumbria (incl. conflict Danish and Norse Vikings).
5) East-Engla, they should start out very very poor (money) and less armies etc., and likely fall to the Danish realm (as in history, the Angle king Eadmund died as hero around 869 AD, iirc.).
6) Mercien, constantly under pressing attack from heathen Northumbria and other pagan realms, falls into pieces. The Mercien king exiled to the Frankish and a puppet king is installed around 870 AD. Not sure, if possible under VC scripting. But start situation should be similar to East-Engla (they will make it longer anyway, as for the many many centers in-game).
7) Wessex, the Last Kingdom. In history they already long had Domnonia/Cornwall under control at 868 AD (iirc. from what i have read, there was only a wild area still under control by Cornish clan leaders in the south of Cornwall). However, for VC the both factions might start at war with each other to keep them busy. Just latest at 870/871 AD (for the game that should be latest 869, perhaps it is possible to script a forced truce until that year ? ... heathen realms attack Wessex with strong and big armies. You know, the "Great Heathen Army" and the still bigger "Summer Army" as reinforcements from the continent fall also into Wessex. Rest Cornwall leaders would ideally ally with the Vikings. While ideally rest Welsh leaders would ally with Wessex, at least the Hywel guy.
Summary:
It would be something like a Wessex history player mod.
One could start as usually, ie. as free-lancing adventurer (building-up enterprises etc.). Perhaps trying to help Mercien, if possible, but mainly friendly/supportive with Wessex*, becoming someday a Vassal called by Aethelred (as happened in my current Sandbox play within 868 AD). If possible, scripting Aethelred's death at 871 AD, and Alfred takes over, who restructures and fights back (just under the players help, who might be already very powerful at that date, ie. one could try to become the Marshall). If the scripting impacts as described are too timeconsuming to do (if overall possible), then just tweaked startconditions could also help for the idea direction, while of course only such script impacts would enable a quasi historical play.
* of course, other player decisions/directions would be possible anyway (fe. support the pagan realms as pagan player)
Just an idea-description.
I did similar historical balance modding of exactly the period for other games (earlier Total War, there many many other periods, recently modded CK2 again for the Viking Age start the 'Big Heathen Army' etc. related codes which is quite easy to do). But as said in the other threads, my hardcore modding days are over, so maybe you would have fun to make a quick and dirty tweak-impact mod like that? Guessing at least, it wouldn't take huge time for you? If not, i might put my hands on it though, although i wanted to avoid such undertakings with VC, just planned ease gaming