Henry V, the Elder of Brunswick
Probably the only really serious choice there. But IOTL, he simply didn't considered the possibility of a Plantagenet inheritence, remaining essentially with his german demesnes.
Long story short, he didn't seem to have actually claimed the english royal title IOTL (it doesn't seem to appears as such contemporarily : but maybe you have different and better sources than I have at hand?). Why would he do so ITTL?
As said above : a putative claim that is not acknowledged, even less enforced, is no claim at all.
Would he be a natural candidate if Alienor dies childless ITTL? Probably much more so, giving the lack of an obvious and legitim candidate among the Anglo-Norman nobility, assuming he takes interest for claiming the kingship of England which is quite possible but not as obvious you make it so.
Anyway, for what matters the actual political situation the moment the PoD kicks in, Aliénor does have the obvious clearer claim as feminine inheritence have precedent in England.
Otto IV, King of Rome 1175
Far too busy trying to hold its own against Philipp of Swabia at this point, and loosing the conflict.
Eventually, what I said about his brother above holds even more for him.
William of Winchester, Lord of Lüneburg(tried to claim crown against John) 1184
Same as above, altough I think he would have a much better chance than Otto if both Aliénor and Henry dies, by virtue not being emperor. Transmission of kingship trough a cadet branch isn't unheard of at this point, and it would be a natural choice (if not condition for acceptance) for both Anglo-Norman but Imperial nobility as well.
Raymond VII, Count of Toulouse
He's probably the one that have clearly the lesser chances to claim anything. And by less chances, I mean he would have none even if everyone else dies : Raimond VIII/IX was at this point not even fully count (admittedlty he was considered as a junior count for what it mattered) but Raimondins barely controlled their own demesne north of Tarn, let alone controlling the plethora of their vassals AND were about to recieve the full strength of the Crusade.
They couldn't even enforce their claim to the Duchy of Narbonne : how in frozen hell would have they been able to enforce a claim over Aquitaine, let alone England?
Of course there were no laws at this point, there were precedents and conventions, which are changeable but did specifically exclude women, Matilda is my example also.
And your exemple is flawed : Mathilda legitimacy was eventually acknowledged trough civil war, not only trugh a de facto situation (as after the Battle of Lincoln, where a significant part of Anglo-Norman nobility acknowledged her as "Mistress of Englishmen" (Domina Anglorum) and most of the resistance to having her being crowned is less due to be a woman, but to the fact Stephen was still alive and didn't renounced the crown yet, and having still enough forces to oppose her in London (mostly due to her poor politics).
I stress this : it's clear from the oath given in 1127 that the issue with her isn't being a woman, it doesn't appear as such in documents (while it does appear for Capetian succession) but from being seen as a lesser candidate overall, having much less ties in England (as Stephen did).
Did the fact she was a woman didn't played? Of course it did, and a lot when it comes to nobiliar and critically clerical alignment : but, and that's an important but, it never was rationalized as such, not creating a clear precedent.
Let us say the two main claimants are the Elder of Brunswick and Eleanor of Brittany, neither children of kings, both grandchildren of Henry II, both grew up in England, one male one female, who do you think would be prefered? The male obviously, unless he was hated or something like that.
I think you're thinking too much in matters of genealogy and (pretty much technical) "claims", and not enough in matter of political situation.
What would have mattered for the Anglo-Norman nobility would have been some stability for the moment being : with the political/military crisis that John managed to create and loose pitfully, the general feeling would have been that Aliénor represented a fair chance of legitimate and immediate succession (being present in England would have been a large bonus compared to other possible pretenders).
Let's rememeber, again, that Welfs didn't show much interest on being the very pretty much technical heirs of England during this decade IOTL. So, a pretender avaible right away and without clear ties with such or such house? It would have been a bloody opportunity.
If something, Aliénor's reign could have been seen by the Anglo-Norman lords as an opportunity to advance their interests more smoothly than it could have been done with John, especially the abandon de facto of the
vis et voluntas principle. I could see, however, the same nobility preparing a later succession, such as making a possible Welf pretender the acting viscount in what remained of Guyenne : finding more advantages with an immediate succession by Aliénor doesn't mean they wouldn't have wanted to prepare an eventual male succession.
Eleanor of Brittany's line was skipped over for succession in England, does that mean she is disinherited, most likely not, but it could be argued that it did, which could put a very small cloud over her dynastic rights in England.
That, on the contrary, is actually a fair point. I could see, would a crisis arise, this being argued.
That said, her claim was just unenforced by her uncle and tutor, which could be as easily be part of the unlawful set of decisions that would be reversed ITTL for the benefit of English nobility : some sort of political pretext.
But indeed, that would be a major argument against her being crowned.