The first thing i would say is not stand alone DLC like Darkspawn Chronicles and Leliana's Song, they don't do anything for the game but give you a few more bits of loot, but beyond that they do nothing for my Warden. I don't know about anyone else but with those two I completed them once then went back and unlocked any loot I missed on the first run, but I have never returned to them since, and I have no interest in doing so. Keep the DLC intergrated with the rest of the campaign, and as much as possible keep the Champion a part of it.
One DLC would be an expansion to the prologue with the character escaping from the battle at Ostagar and the trip to Lothering, you could even throw in a few cameos from characters like Alistair, the Warden, Sten, Duncan. If you are using a mage Champion you could have control of Carver through it (or at least until you reach Lothering and the rest of your family). The DLC could then end with the escape from Lothering where the game would give you the option of carrying on into the main game with whatever you got from the DLC. So you would end up starting the game with a few more levels, some loot, and you could even have a few events from the DLC carry over, with you meeting people from the DLC in Kirkwall.
Another LI would be interesting, preferably female as their is a shortage if your character is a straight male.
Another DLC that would be intersting would be set between the time of the end of the campaign and Varric's capture and interogation. A possible story would be that one or more of the CIty-states decide to make a move to grab Kirkwall after the events of the campaign, and you as the champion are the one asked to help deal with it (whatever choices you made at the end of the campaign you are basically the only person of power left in the city so it would make sense that you would have to deal with it.) The DLC could have you going to new places, other city-states to look for allies. You could even go to Starkhaven where you would experience extra things based on if you have the Exiled Prince DLC (and your choices during it). That would extend the duration of the game closer to 10 years (of course then people would complain that they have to pay to experience the full story but who cares about them), it would also fit into the rest of the game's theme of your character's rise to Champion. It should also not effect Varric's interview enough that it would need to be changed very much.
Those are just a couple of suggestions, the first could be a pretty small DLC, the second could be anywhere between a large DLC up to a small expansion depending on how much can be thought of to put in it.
But there have been a lot of other good ideas such as having a trip on Isabela's ship, but as I said I really think that you should stick to LDC that fit into the campaign rather than stand alone ones that don't extend the length of the main campaign and have no real replay value.