Running Dragon Age 2 in a roleplaying game can easily be summed up: use Pathfinder or Dungeons & Dragons in a rich campaign setting.
Oh, you want more?
Well, there's a few main things to a game like Dragon Age 2. Identify your themes and stick close to them, interweaving them throughout your story. In the case of Dragon Age 2, the themes are segregation and racism - their causes, benefits and flaws. There's also a strong thread of disease and contamination in both this game and the first one - Lyrium, Darkspawn taint, Mages letting in demons and becoming abominations.
On that note, its important to really introduce that whole 'Magic has a downside' theme that is more prevalent in books than roleplaying games these days. In the game world, mages must be rigorously trained and controlled lest they succumb to the whispers of demons in their dreams that wish to get a foothold in their minds in order to take on their bodies. Even those who've undergone the training also sometimes fail the 'test' where they must survive a trip through the dreamlands (referred to in-world as the Fade).
How would you incorporate this in a roleplaying game? Pretty easily unless you want to throw the risk at the players again (which is not recommended outside of games built for it like Dark Heresy). You could just throw in a few sub-plots where an untrained mage goes nuts and starts casting their spells willy nilly. Or they use too much power and are 'possessed' by a demon and become an abomination where you simply replace their statistics with those of a monster.
If you want it to be a risk for player mages as well then you have the issue of balancing the chance of it against the worth of actually playing in that class. I'd suggest either ruling out all PC mages, making PC mages immune for some reason, or allowing PC mages to have survived some sort of training which means that you can dangle it in front of them as a temptation but never allow them to grasp it (lest they lose their character). You could build in elements of partial possessions and what-not but you'll need to start looking at Templates and House Rules and that can be tricky and often requires a lot of tinkering to get it to really work.
A rich campaign world is very important if you're going for the Dragon Age feel. There needs to be a lot of complexity and interaction between the various factions and the cultures need to feel distinct. World building is far too big a concept to go into in this article so I suggest you Google fantasy world building or head on down to your local library for any How To Write Fantasy books.
Darkspawn aren't as big a thing in this game, which makes sense considering that your various party members aren't immune to the taint (except for one), but they're still an important Always Evil race that adds a particularly malevolent twist to the world. If they bite you, you get tainted and slowly become a Darkspawn unless you have a very strong constitution and are tainted through a specially made Warden's potion. The process of transformation is very painful, by the way.
|Different standards of beauty? Nah, all those women meet my standards.|
It gets trickier if there's an upside to such a corruption such as if you gain greater strength or resilience but are slowly but surely becoming a monster. Still, such a thing can normally be represented by a bonus to some physical attributes that slowly accumulates over time. I wouldn't give them any special abilities due to the Taint until its won them over but that's just me. You might want to show them fully transforming into something wicked and powerful or you might want to represent something that simple attribute buffs just won't do.
In the end, it's up to you.
A campaign based around Dragon Age 2, or including elements of it, should appeal to Explorers less than the original game because this one is far more on rails and you've only got a set area to explore. Having said that, urban exploration still has a lot of value to it and so long as you allow the place to open up a little and ensure that the different areas don't blend together you should have them in the palm of your hand. Tacticians will enjoy the different range of skills and abilities the group brings to the party as it has a very Dungeons & Dragons-style group of healers, fighters, mages, and rogues, and Tacticians can often get a lot of mileage out of setting up the right cascade of effects so long as the other players are willing to listen. Action Heroes will enjoy the high-stakes drama of an adventure that sees to major power players (Templars and Mages) at each other's throats. They'll get to feel super-important in the non-action elements and powerful in the action spots.
Investigators can get a kick out of any of the quests that require a bit of clue combing or lead following. Pose a question to them, in this case a conspiracy, and give them a chance to find the solution and they'll be a happy chappy. Unfortunately, they may get a bit bored during the spaces in between that don't seem to advance their investigations (such as random thug bashing). Communicators will love the colourful characters and the chance to explore different cultures in an urban game but may resist being moved on too quickly. If everyone is enjoying a cultural exchange or a particular NPC, let them engage with it a little bit longer than you normally would before moving things along to balance the combat and action segments for them.