I just meant cultural ties, which I figured might lead to marital ties which then, at least if the system of noble houses and feudalism evolves as it did almost necessarily mean political ties. Given the dynastic politics of the middle ages, I could absolutely imagine a dynasty coming to rule over wales and brittany.
It's very unlikely for any PoD before 1000 or even 1100. Before that, what makes a king is less the inheritance (altough it gives a great legitimacy) than the nobility acceptence (in a more or less formal election for some states).
Given that the transmaritime prievilegied links between Cornwall and Britanny (Dumnonia) are totally part of Early Middle-Ages, it makes it unlikely).
In a post-1000/1100 PoD these links, that have been exaggerated by celtomaniac historiography, are dead meat. The Bretons are, since the VIII far more turned in the direction of continent rather than Britain.
You could no more argue of Cornwall/Britanny ties for the searched goal.
And such a dynasty could again in turn come under the domination of, or might simply lose it's possessions to, one french dynasty or another.
Again, not only the nobility of a country, foreign to a frankish or continental dynasty would be hostile to the take over of someone being stranger, but if he's king and want to just rule the two countries, especially a transmaritime one, he better be prepared to regular rebellions.
Remember how Danish were eventually rejected as a foreign body, and how long it takes for Normans to be accepted.
The golden rule is : no random marriages, dipomacy by martial ties are systematicaly made in aeras where it's something interesting AND where the married side's family could intervene.
But I think it's rather obvious that the only way this could somehow result in a french ruled, or dominated, wales is if england doesnt manage, or try to bring wales under it's control.
Unless we're talking of West Wales, aka today's Cornwall, you have no maritial, cultural, diplomatic, economic or whatever-ic privilegied tie with Wales (Cambria). At least, not more Britanny had with Francia, Frisia or Anglo-Saxons kingdoms.
First, I said england, not britain. Second, by that I meant england not existing as a political entity. At least not a unified one capable of coordinated conquest of it's neighbours.
Ok, England. Acknowledged.
Let's say that the Godwin dynasty is simply butterflied, why not. If the anglo-saxons are even more badly organised, divided in more kingdom, you'll only trade an Anglo-Saxon England to a full Danelag England. Less powerful kingdoms would be easy preys for Danish conquest. Seeing how much they were surronding celtic aeras, i would bet that it could even led to a earlier conquest of Corwnall and Irish-sea countries.
My thoughts were basically: IF wales is to be french (by whatever plausible or implausible means) that pretty much requires that england doesn't grab it.
Not mandatory. As you said, the less implausible way is probably not the conquest, as nobody had the possibility to do it by not oversizing the income expected of the conquest.
The problem, again if we're talking about Cambria and not "West Wales", is how Wales are supposed to interest enough the Frankish or the French nobilty to make alliance, or martial ties with. Even in the wonderful (litteraly) case where whe have divided anglo-saxons kingdoms and no all-triumphant Danelaw, what is "interesting" is the south or the east and not the west.
For england not take over Wales it would either have to be in some way significantly weaker than it actually was, under the control of some continental ruler with no great interest in dominating the rest of the british isles, or completely nonexistent as a political entity.
Expect that you've probably made the position of Cornwall and Wales even more precarious : each kingdom, if we don't want them interested into the union, must have his own interest elsewhere.
By exemple the Sussex and Essex would be more present in Frisia, Mercia and Northumburia or danelawized or interested about the conquest of Lothian. And the Wessex, by exemple, would be interested about the raid and eventually the conquest of Cornwall, then Wales. I don't mean that will result in automatic conquest of Cambria (for Cornwall, at the contrary, i mean that exactly), but it would totally weaken Wales, making even more a marginal land, even less interesting.