One problem I see with sticking with the Inquisitor, even if there is huge story potential there IMO (DLC will come of course), is that it'll set a precedent. If they stick with the Inquisitor for DA4 and then change to a new PC for DA5, people will complain twice as loud and twice as long about it. People are still complaining about not playing the Warden. Heavy risk, but you can finish off the meme yourself.
Really I think I'd have a better idea of if I'd want this after we've had some of the DA:I DLC.
Also I think it could be interesting to have a new PC who is part of the Inquisition but not the Inquisitor. Someone who perhaps rises through the ranks and perhaps even works with the Inquisitor later on (ala Hawke with the CC). After all the Inquisition at the end of the game is an organisation spanning several countries, depending on wartable choices, even Tevinter and Nevarra.