My biggest misgiving about the way in which the writers have handled the end with Dorian is that they have left the door open to bringing him back without the Inquisitor in a subsequent instalment of the game. Now you can view that as good news because it means more Dorian or bad news because he's no longer with the Inquisitor.
Just because the Inquisitor won't be there (if a future game with a new protag) doesn't mean he is no longer "with" the Inquisitor. In DAI Leliana, Alistair, and Morrigan are all still in love with your Warden if your import worked correctly. And of course we see that Hawke and Varric both directly reference Hawke's LI.
Personally, I'm already in the correct headspace for a long distance relationship based on my Warden and Alistair, so I wouldn't be too upset to see Dorian again. If he throws out a line or two about his boyfriend, I would be pretty satisfied.
It is a bit how I felt when there was no Zevran in Awakening and then he turned up in DA2 and was cheating on my Warden. I was thinking, this isn't Zevran, it's an imposter.
I know that this was frustrating for Zevran romancers, but as tklivory explained, this was a known bug. Zevran was NOT supposed to cheat on your Warden if you imported a Zevran romance DAO play into DA2. I know the DA Keep isn't completely perfect, but it is supposed to prevent problems like this from occurring in the future.