What romance for my next PT?

For a blood mage, I'd go with Merrill (or Izzy, but you've already romanced her). It's not really dealt with in-game but both Fenris and Anders object to every other use of blood magic in the series so I can't realistically see either of them shacking up with a blood mage.

Blood mage? If you can make it work with Fenris it'd be so intense.

Since Isabella is out, Fenris or Anders. Both are great romances.
I'm not a fan of Merrill, despite loving Eve Myles, the voice actress

Merill for the blood magic

Fenris if you love the angst like I do, haha

I honestly wanna know how many times Mark Meer cracked up doing some of that dialogue.

Don't go for Fenris unless you're prepared to be an outright dick to him even outside heated political debates to rack up rivalry points.

I tried to do the rivalry thing with him, but even just being honest and civilized with him during tender and totally nonpolitical moments set me back too much to get his full romance, even by hammering the mage freedom button a bit more firmly than I'd intended.

Ended up deciding that he was too damaged to have a real relationship and went for a casual "mutually good influences" thing with Bella instead, ultimately not finding love in Kirkwall in any real sense. Because the friendship/rivalry system should have been thrown in the gutter and put out of its misery along with the career of whoever cooked it up.

All that said, I'm liable to sacrifice Hawke to save Loghain in Inquisition anyway, so it's really just one less lose end. *totally not passive-aggressive*

Anders is absolutely not irredeemable. How? I can understand if someone doesn't like his romance – this is a hard one (like Fenris'), and not for everyone. But what he did in this world is maximum grey – but compared to the others he's the good guy.

Isabela probably a good choice to you. (If you can redeem her after, that she goes away with the Koslun and because of it, one of the causes of the Qunari war here.)

In this world Anders is still the good guy – even if his act is grey. Far not irredeemable.
Fenris' act is weird – if Hawke helped him to eliminate Hadriana and Danarius. But he just follows his fear, if Hawke doesn't listen him (neutral zone).
Aveline also leaves Hawke for Meredith – with special circumstances, true, when she faces with Hawke on the battleground, she, says, she owes Hawke with one (her) life, so, the debt now has been paid, drops her weapon and leaves.

The friendship/rivalry system has many flaws, one of them that if you in the neutral zone, game evaluates that Hawke doesn't care about the character.
So: Hawke can help Fenris, still stuch in the neutral zone.
Yes, but if Hawke doesn't care about Aveline, she will follow the order – and the order is not really the revolution, even if Meredith's a mad criminal abomination: the Templars embodies the law...