Hmm...why Morcar? Ever since this got raised I've been trying to find a history book which details exactly what happened in the Leofricsons' rebellion, but was Morcar the protagonist? He was elected as Earl of Northumbria following the popular rebellion against Tostig Godwineson, of which he was a part, but it was Edwin who threw the army of Mercia behind the rebels. Harold Godwineson, before he was King Harold, also came to their aid and utterly defated Tostig...but through all of that, Morcar wasn't all that influential.
AFAIK, Edwin was the ambitious one of the two brothers...the only logical reason I can see to favour Morcar would be that Northumbria was naturally more rebellious. Hence why I'm trying to find a book on the rebellion, but I believe it was started by Edwin.
Northumbria wasn't subject to the harrowing of the north until 1069, when a completely separate effort was made by Scotland and Denmark to put Edgar Aetheling on the throne. By this stage, William the Conqueror realised that the combination of Northumbria's natural streak of independence, distance from his own power base and proximity to potentially powerful enemies caused him to realise that the most sensible solution was to destroy the place in order to deny his enemies access to its resources that could have been used against him.
Basically, although Northumbria was involved more in the rebellions and suffered far more, this was mainly after the Leofricsons' rebellion, and more because of its strategic location than because of its ruler.
Anyway, in terms of CK mechanics, the Leofricsons need some kind of claim on territory that they can pursue against King William, or they'll never actually declare war against him (ie, they can't pursue Edgar's claim). Gytha Godwineson and Sweyn Estridson are tied up in this, too...
I've made various comments on this previously, but I'll collate my thoughts here on who should have what claims. Reasoning follows further down:
Edgar Atheling: (King of England)
Gytha Godwineson: any selection of (County of Hampshire, County of Salisbury, County of Bristol, County of Dorset, County of Somerset, County of Devon)
Sweyn Estridson: (County of York, County of Durham), also arguably you could include other areas of former Danelaw, which would include any selection of (County of Lancaster, County of Derby, County of Leicester, County of Lincoln).
Edwin Leofricson: any selection of (County of Oxford, County of Gloucester, County of Warwick and County of Leicester) and arguably also (County of Hereford and County of Northampton)
Morcar Leofricson: (County of Leicester, County of Lincoln, County of Cumberland) and arguably also (County of Berwick).
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Reasoning:
Edgar Aetheling is pretty obvious, given that he was appointed King by the last Saxon Witan before submission to William the Conqueror.
Gytha Godwineson, also Gytha of Wessex, daughter of Harold Godwineson is not really a true claimant to the throne. Having no sons when Harold Godwineson was killed at Hastings, she was dutifully passed over in the succession order. Given that the Witan elected Edgar Aetheling, her claims would now only be logically valid on her father's former lands in Wessex, which cover the counties listed.
Arguably, you could also include claims on the Duke of Norfolk and Duke of Hereford, as her father had held them. It depends how much of a key player you want to make her, but historically she was very, very minor.
Sweyn Estridson, IIRC the uncle of Gytha Godwineson, again has only the loosest claim on the throne of England, as it passes through the female line and through marriage only. Sweyn would have valid claims on some lands in former Danelaw in England, having a slight hereditary right to them. York and Durham are the obvious choices, being the bastions of Danish culture in northern England.
Edwin Leofricson would have claims on the former lands of the Earldom of Mercia. In-game this is rather confusing, because representing him as the Duke of Lancaster already throws him a few hundred miles too far to the north, IIRC. However, Mercia's former ownership of Oxford, Gloucester, Warwick and Leicester was still recent enough for a direct claim to be valid. Arguably, you could include claims on Hereford and Northampton if you want to trace Mercia's borders back slightly further, but that would be stretching things.
Morcar Leofricson, in his capacity as Earl of Northumbria, would have claims on the disputed borders - Leicester, Lincoln and Cumberland. Arguably, you could stretch that up to include Berwick as well.
What this does is give the key players historically supported claims which are also, IMO, fairly balanced - especially if you make sure that Gytha Godwineson's claims are kept low (and have her resident in Denmark, where she had escaped to following the conquest...). There is a possibility that a faintly historical situation could arise, with the Leofricsons trying to expand their direct territory and, in the event of dramatic success, be able to put Aetheling on the throne, since he resides with them.
If Malcolm of Scotland marries Margaret Aetheling, then Edgar should join him if he becomes exiled from England (historical), and then he can follow his historical claim. Sweyn Estridson may choose to pursue his claims, possibly aiding Gytha Godwineson.
Edgar Aetheling, meanwhile, remains the King candidate in desperate need of a Kingmaker, and Margaret Aetheling (who *was* a key player) the link to securing his claim on the throne of England for your dynasty...providing no opportunistic sod prevents you from doing so by marrying his daughter to Edgar.