I am happy that the Warden is not the in game at all. By the end of Awakening my Warden was a god. He probably could one shot every high dragon of Thedas. Giving him a cheap cameo just because is a big no for me.
What would be cool is having a shot at the Warden in the far distance with five high dragon corpse at his feet. Or maybe even controlling him myself in a future expansion/dlc. Starting him has a level 25 up to 35 etc. He does not even need a voice over. I would play/purchase that.
But just has a cameo ? No. The Warden deserve better than being a cheap cameo.
I have to hope a little that you're not actually serious about the Warden. He couldn't one shot anything major, in terms of the story. He always has his allies by his side to help. He might be called one of the, if not the most powerful widely-known individual in Ferelden, but that's about it, and that has its limits too.
He can kill broodmothers and high dragons and an archdemon with some help, yay. That's not unheard of. At least several others in Thedeosian history have done similar.
Killing dragons is something that many did. Its what Nevarra is even known for. Its nothing super super special (but it is prestigious, and until now rare because the dragons were even being killed off and thought extinct).
Killing dawkspawn is something many did. Its what dwarves are known for. I believe at least some of them (Legion of the Dead?) is even aware of broodmothers and killing them, and definitely the Grey Wardens are.
Our level is not really relevant. And lots of say, mages, can learn lots of spells and that's not Warden exclusive.
The Warden-Commander mayyy be one of the strongest people in known Thedas, but it isn't like he's an actual god. IMO Hawke is stronger as an individual (if you strip away the Warden's party support and special training paths and skills), and the Inquisitor heads into demigod territory where neither tread (being a Warden is special, but not that special).
I agree that he shouldn't be a cheap cameo. But that's why I'd want him to be a major presence and influence in content, not just an appearance.