Welsh exodus to USA is an obvious one, but I think a path that has been tread before. Is it just me, or does the +prestige, +legitimacy, small size and dank religious + idea bonuses suggest Wales could become a secret Holy Roman Welsh Emperor strat?
Think about it: From Wales you could eliminate the protestant centre of reformation in the British Isles and thus eliminate one of the toughest protestant strongholds. With bonus prestige and legitimacy you'll never lack for bonuses from other states wanting you to be Emperor (once you're big enough), and if you suck up to the Austrians before the Dutch leave, you could theoretically add yourself to the HRE or else just go the long route, become Emperor and so add yourself that way. With bonus to religious ideas and conversion you're a prime candidate for a German and Scandinavian protestant chemotherapy edition, eliminating centres of reformation with missionary strength no one else possesses (except maybe the Byzantines, but they won't have survived this long anyways so no one cares). With your idea groups stacked religious and humanist, you're basically immune to the reformation!
You also have a natural reason to reclaim France as the natural celtic descendants of One True Celtic Francetm. And if you become HRE and defeat the protestants and Dutch, revoking the privilegia cements your hold on the English Channel trade node, as all other trade income goes to your vassals instead of rivals. The only downside I think, is that despite your increased force limit, manpower may still remain an issue, with regards to winning a league war.