I know that if Bio puts that quest in just to bring the Warden back, he won't find anything.
Put another way, I think your argument goes at this backwards. The only way to make this work is to sell to Bio -- you can practice on us -- that revealing the source of the darkspawn in the next game is a good plan on the merits. Then bringing the Warden back looks like a natural thing. Right now it's pretty transparent that you're flailing around for a pretext to bring the Warden back, and the plot of the next game is secondary to that desire. The problem this introduces is that you're trying to sell the idea to people who are more interested in the plot than they are in the Warden.
Who said anything about a quest? A lot of us would love to see him/her as the protagonist next game. Definitely wouldn't end in not finding anything, and versus wandering the hissing wastes (admittedly pretty) which doesn't have a single npc with dialogue, it sounds like the makings of a fantastic story based mission.
And tou're dead wrong, it's for plot that I want him back. Our past decisions carry over, our former companions are carrying over, and a new pc has no investment in any of that. The
warden has a personal involvement in those events, and you'll get far better story out of having the characters involved actually involved, as opposed to being talked about second hand. The
warden could explore the relationships he built in origins, for the inquisitor it's like "hey, nice to meet you".
I don't need to meet my friends for the first time every single game. It'd be nice to be recognized, and have commentary about the things that happened in previous games that actually applies to the characters discussing it. Leliana was furious in DAI that one of my
wardens almost killed her at the urn of sacred ashes, it was great, and watching them face off? That would be amazing.
Besides, Corypheus is just one of the magisters, pretty sure the architect is another, and now that the Qunari are basically really tall, really pretty elves with horns versus the beastial fanatics they were in past games, the darkspawn is the only intimidating enemy left...
And seeing orzimmar and Kal shirok would be nice. Sick of big empty forests.