I don't think there is any right way of involving the warden outside of DAO. As far as I'm concerned, I would rather the Warden be killed off than persist into DAI. I say, let go of that character, at this point the warden is just an anchor weighing the franchise down.
Hardly.
There are a ton of ways you could have the Warden still a part of the world in a way that seems natural without seeming forced. I have no clue to what extent BioWare will do anything with the Warden in DA:I, but my concern here is that they're going to treat the Hero of Ferelden that lived in just the same way as one who died- namely either by killing them off offscreen or otherwise rendering them conspicuously out of action.
And what would make that seem jarring to me is that they're seemingly bringing back so many other NPCs from Origins that could be very closely linked to your Warden (Morrigan, Leliana, Alistair) and they're even bringing back an old PC in Hawke. There are a lot of reasons they could give in game as to why the Warden isn't showing up but when they bring back somebody like Morrigan and Morrigan and the Warden's kid but not the Warden? Eh, I don't know.
As far as the OGB goes, I always kind of hoped having your Warden go through the Eluvian with Morrigan meant your Warden would have some role in their upbringing. Maybe the Warden is off on some mission or something during Inquisition, which could work, but I'll just be very curious to see how they justify the Hero of Ferelden's absence when they have no problem bringing every other important PC/NPC from Origins and DA2 back in some capacity.