I'm sure all of these have been brought up before, but the tropes that I can think of are the "recovery from lost love" trope, the "love can mend a broken soul" trope, and the "bisexuals are always promiscuous" trope.
Recovery from Lost Love: Carth, Kaidan, Thane, Sky, others. Usually this is a straight male, but it is also possible for the character to be a gay or bisexual male. Would be less of an issue if this happened with women from time to time, but still... When we're talking about romances, a Lost Love means that the character in question doesn't love the protagonist most, because there's always that Lost Love from whom he (or more rarely she) will never recover. Now I'm not saying I'd want a romance that's pure as the driven snow, or another virginal Alistair type, but the long-dead/lost love/family is problematic because it's been done so much. How about a few relationships that simply didn't work out--for both male and female LIs? Not every past relationship has to have ended in tragedy. Sometimes, people just fall out of love, or they realize belatedly that they were never in love in the first place. Or, sometimes people date and take a long time to "click" with someone.
Love Can Mend A Broken Soul: The biggest character that comes to my mind is Jack; however, Zevran also counts to some degree and perhaps also Morrigan. So you encounter this character who's basically been through hell, mentally, physically, or both. In Jack's case, there is even implied rape. Rather than getting these characters counseling (in worlds where that's a thing) or being a good friend and shoulder to lean on, the protagonist ends up romancing them--even when it would make more sense, in context of the situation, to just be a friend and help that other character work things out. They might never work things out. Jack in particular never struck me as someone who ought to be in a romantic relationship--too many conflicting emotions in her about sex and romance, due to her past experiences. But, magically, after having healing sex/relationships with the Main Character, the Broken Soul is suddenly okay, or at least better than he or she was prior to that magical healing sex. Now, troubled pasts aren't a horrible thing in and of themselves, though it is preferable they be used sparingly--and it's not entirely wrong to have a relationship with someone who's troubled. The problem is that when someone has that many issues, they aren't going to be solved by a romantic partner having sex with the troubled individual. Having a considerate, caring relationship may help, but things would still be complicated, and the troubled person may even have conflicting feelings about the relationship even if it is a healthy one. If there's a troubled character in a relationship, it may be a bit more realistic if it plays out something like Anders' did (yeah I know Anders is all kinds of trouble, BUT). In other words, there may be a lot of love there, but the troubled person is still troubled, and may or may not need more help and support than the partner protagonist can give.
Promiscuous Bisexuals: Zevran. Isabella. Iron Bull. Seriously? I know that bisexuals aren't all promiscuous, and I'm a straight person. And, there are a lot of promiscuous people who are straight, as well (maybe a higher percentage than bisexuals, but I can't be sure of that). While 2/3 of the promiscuous bisexual characters I can think of were male, and that's also an issue--not all men are promiscuous, and as a straight woman I can tell you that the last thing I want in a man is promiscuity--I don't have a big enough sample size to say for sure that they will be unequal. Still, it's a problem. Many bisexuals are quite monogamous, or don't find that many people attractive, or ARE attracted to many people but still manage to be loyal to just one person. Zevran also has the attached issue that he claims to have been "made" bisexual, but that's a separate matter. It wouldn't be an issue if it were only one bisexual who'd been promiscuous, but now we're getting a pattern. Can we please break this pattern after Iron Bull? One or two just indicate that's how that character is. Three over the course of three games is problematic. Have a straight character be promiscuous for once--maybe a straight female, since the LGBT community's had enough of it, and I, as a lady, am sick of the Promiscuous Man trope.
And since I mentioned it...
The Promiscuous Man Trope... Zevran, Iron Bull, possibly Anders (uncertain of that, though he's a flirt for sure), Jacob (to some degree, since he cheats). Probably others I don't remember. This has fortunately not been as common with Bioware as some other companies. HOWEVER, it's worth mentioning to prevent it becoming too common. Society likes to believe men would want all kinds of pretty girlfriends hanging off their shoulders, so women are presented with a guy who sleeps around with a lot of other people (in the case of Zevran and Iron Bull those people may also be male, but in most cases in the media, he of course would have many girlfriends as lovers) or even just flirts with them. When the female protagonist romances him, he's magically corrected his ways and she can become his One and Only. While I know this is the ideal and sweet and sappy and romantic, it just isn't believable, and I can't help thinking my character should wind up with some STD for being with a guy like that. And then you have cases like Jacob, where he seems loyal, but is apparently promiscuous... so you get cheated on (which hasn't happened with any female LIs, thus only straight female Shepards lost an LI in this way). That's not fun in real life, and it's certainly not fun in a game. Maybe a better way to deal with promiscuous characters is to have them be open about it, and maybe have the type of character who separates romance from sex. "I love you, but I really like sex. It's okay if we have sex with other people, because it's just sex and we know we love each other and still return to each other no matter who else we have sex with." But that's a complicated type of relationship (note also, it's not my thing, but I could see playing a character who thinks that way) and may be a bit too complicated for the sake of a game romance. I think what I am saying is when we generally expect monogamous pairings from the LIs, it'd be nice if they either returned the favor or made it clear that they had no intention of being monogamous, at least sexually (Isabella is likely a good example of this, but I never romanced her so I can't say for sure--maybe she ended up monogamous as well?).