I can't think of any realistic scenario where Wales expands beyond its current borders in the long term, but a strong enough sense of unification to resist being conquered or at least annexed into England should be possible. One such method would be finding a way to cement the rule of Dafydd ap Llywelyn, the sole chosen heir of Llywelyn the Great. Otherwise something involving his grandson, Llywelyn ap Gruffydd might be in order as he came close enough that he was actually recognized as more or less a sovereign Prince of Wales by the King of England himself. If perhaps he offed his older brother Owain to keep Gwynedd more stable, and didn't alienate Deheubarth by rewarding its prince's rebellious brother with land he might have maintained a pretty strong grip on the whole Principality, enough to be able to pass it down to a sole heir like his grandfather tried should it last long enough to not be conquered by England before he has a legitimate son.
I toyed with making a TL along these lines, although it didn't get very far. The POD was that Llywelyn ap Gruffydd ended up having to kill Owain after the Battle of Bryn Derwin and so took a more ruthlessly pragmatic approach to politics after the incident, tolerating no more trouble from his younger brother Dafydd, keeping Dehuebarth on his side, and in his more secure position he sends more support to his English ally Simon de Montfort, who with the extra help forces the English royalists at Eversham into a disastrous Pyrrhic victory which makes the Second Baronial Rebellion spiral into a Second Anarchy, further giving Llywelyn a free hand to stabilize and rule Wales while England descends into chaos and upheaval as the remnants of the Baronial forces and some allies continue to resist against the newly crowned King Edmund.