I'm not sure dramatic relationships (i.e. ones in books, films, games etc) are always meant to be "yes, this is a good and sensible choice for who to spend the rest of my days with", really? I mean, obviously there are examples where the "passion and drama" is more "creepy obsessive weirdo meets person too impressed by their nice bottom/fancy car/commitment to radical politics to notice the creepy weirdo part"* but I suspect there wouldn't be threads thousands of pages long about a love interest whose romance culminates in organising a mortgage and a trip to the Hightown Haberdashery to pick out curtain material. Okay, Alistair in DAO is the closest to a Normal Person, and he's very popular (I'm a big fan myself) but even for the Warden who gets to marry him, well, becoming Queen Consort at the end of a civil war and only surviving ending the Blight by convincing your only-recently-deflowered boyfriend to impregnate your loony witch friend (again, big fan of her, too) with an Old God Baby isn't exactly a recipe for fun, fluffy settling down...
Also, while obviously people get to pick who they have relationships with and you get to not want to be with someone for any reason including "they breathe funny" or whatever, being mentally ill/exhibiting signs of trauma isn't necessarily a red flag and lots of mentally ill people are in relationships and are good partners, as much as any other person, anyway (speaking as someone who was emotionally abused by my first long-term boyfriend who is a Normal, Healthy Man (while I am mentally ill, have a developmental disorder *and* am physically disabled - although he still (over a decade later) refuses to accept this part is true - which he variously used as reasons why he was a Saint for Putting Up With Me because I was a disgusting lazy freak)). Mind you, when Anders starts going totally bonkers towards the end, a responsible lover, or friend (I tend to see the 'friend/rival' label as the *type* of friendship, not whether they're friends or not - someone who's still around the middle is maybe still just this guy you see sometimes, someone at either end who still talks to you is a friend, IYSWIM) would help him to find someone who could help him, not go 'oh, we've been collecting weird ingredients, I guess I shouldn't look into whether they might be components for explosives or anything... sure, I'll help you sneak around the Chantry'.
I haven't romanced Isabela or Merrill yet, but I think they and Fenris (probably not Anders, really) have potential for long term relationshipness, no, they're probably none of them curtains and mortgage material but *neither is Hawke*. Imagine Hawke marrying some tedious noble who initially finds their adventurous ways charming, but quickly turns to embarrassment and gradually scorn when it becomes plain Hawke's still going to be going down the Hanged Man to drink with Varric (okay, in a world where they don't all have to disappear after the end of the game) and teaching that weird elf to read and helping that other weird elf try to find a safer relic of her people to restore, I mean, at least the Guard Captain's a good influence, but she's still a Fereldan...
*examples: Christian Gray (50 Shades) - not for the BDSM, but the manipulative stalker part; Edward Cullen (Twilight) - not for the vampire thing but the manipulative stalker part, Anders - not for the... you get the idea. Okay, the vampire and mass-slaughterer-of-innocents-sort-of-abomination parts, too