My opinion? I'm pretty sure Fenris would want nothing to do with dominance/submission play, or restraints beyond perhaps using a discarded belt; certainly no hardware, and collars are
right out. While it is definitely true that reactions to trauma are many and varied, and every person's way of healing and coping is as valid as any other (provided it isn't a method that will hurt them even worse, e.g. heavy drinking for years, using drugs, or flirting with suicide), I don't see Fenris as wanting to reenact any moments from his slavery, whether with himself as the master or the slave. He might enjoy being dominant, but he would also enjoy his lover pushing back, a meeting of equals.
I think his idea of 'spicing up' would be more like creative uses of wine and fruits, dirty talk, possibly semi-public places, etc. Of course, I also don't think that he would get bored of things they could do in the bedroom without props very quickly, either. Isabela might push him faster than Hawke, both because they're more 'casual' and don't have the emotional connection initially that would make things more intense, and because she's already experienced a lot of things and enjoys certain kinks and would want to try them with him. Not to say Hawke can't be experienced (most of mine certainly weren't new to sex), but generally, between Isabela and Hawke, I think most of us can agree who's the more expert between the sheets. Or against the wall. Or over a table. Or in a Chantry pew. . .
But really, with Fenris, just making love to Hawke would be immensely novel and satisfying for a long time, I think. He's free, he's with the person he loves (even if he doesn't say those words, he says three others that mean
so much more, since they're from
his lips), who needs more spice? That already
is spice. Three years of simmering love and lust would make for a long-burning flame, I think.
But of course, all of this is my opinion! My headcanon, as it were, though there's iterations for various different Hawkes, and for Isabela. Other people may well disagree, and that's perfectly fine.