I don't think a dual protagonist would work at all, that would totally ruin the game for me actually.
... And cutting away or postponing what has been established in DAI somehow doesn't ruin the game? Dragon Age is not TES - it naturally follows what has been established in previous games.
DA4 would be a good opportunity to give a fleshed out Tevinter perspective, although it would also be a good opportunity to have a disabled character being important to the story as well. At the end of Trespasser one of the dialogue options even has the inky saying 'my adventuring days are over'...but then in the last scene they're pictured in Haven with Leliana and Cassandra making future plans. I think perhaps they might serve as an agent like in Inquisition, but I don't think they'll have a very big role in the next game.
The "my adventuring days are over" exists only in one dialogue option - the aggressive one literally has "now excuse me - I have the world to save. Again."
I somehow doubt that Inquisitor's involvement will depend on which dialogue option we choose at the end, or even if we choose to disband or not - not with the last scene being Inky planning a visit in Tevinter and with hints all over the epilogue cards (especially befriended Sera's) that the whole "retirement" is just a ruse to placate public.
I mean, it is implied by the ending itself that Tevinter becomes important for Inquisitor in plans to stop Solas - that itself makes them important, and I can't see how they'd scale down their involvement without it feeling cheap, if we only end up with a cameo or worse - a chain of letters or proxies.
I don't think Solas' and the Inquisitor's story is done but it's pretty hard to see it being a happy ending at this stage. I reckon Bioware might do a cut scene like they did with Hawke, but imo I don't think the romance would ever really work.
It seems to me like Solas has way more backstory than he's told us about him and Mythal too.
At this point this is not about a happy ending - Trespasser itself strongly implied that it's Inquisitor's job to either try and stop and perhaps kill Solas or try and redeem him. They wouldn't be using words as strong as that if they didn't have something planned for Inky... maybe not as much as they have for Solas, but certainly more than "just a previous protagonist". Trespasser effectively gave them a new purpose.