I'm looking for a well balanced love triangle. The thought of a wrong-person-right-time rival seems to be weak because readers are most likely not to root for the him/her if the wrongness is obvious.
In her writing book Plot, Ansen Dibell discusses the technique of mirroring characters - two characters who are alike in many ways, and different in others. This gives you a "compare-and-contrast" effect, where the contrast between the two characters naturally creates significance and tension. This excerpt via Google Books has a lot of the section. She gives the classic example of A Christmas Carol, which compares Scrooge to past and future versions of himself - and to other characters, from Marley to Tiny Tim.
It sounds like you're looking for something similar: a good contrast to the right-guy-wrong-time; another character on a similar spectrum which will naturally create tension with your established love-interest. So, what elements can we play around with here?
One option would be to have a different right-guy-wrong-time, who represents a different "wrong time" than the first does. If one guy would be perfect but only after the protagonist gets her career together, the second could be someone who would've been perfect if only she'd met him before she started. This turns the tension into the tension between different "versions" of your protagonist's self image - is she more at home with her past, or with her future? Can she reconcile between either of them and her actual present state?
- (A similar option would be two right-guy-wrong-time, each "corresponding" to a different possible future - for example, one that'd be perfect IF her life calms down and settles into a routine, another that'd be perfect IF it speeds up to an even more frenetic pace. This way it's a clash between "possible futures.")
Another option would be to invert the problem with right-guy-wrong-time - instead of somebody who will be terrific but not now, somebody who is terrific but at some point later, he won't be anymore. This gives you tension which is more future vs. present; immediate gratification vs. patience; opportunism vs. holding out in the hopes of still finding something good later on.
Let's try inverting along a different axis: what if the second guy is perfect for the protagonist, but for him, she's the perfect-girl-wrong-time? She finds herself at the receiving end of the same problems she has with Guy #1. Here, the tension is rather different: it becomes about what she can demand of a romantic partner, and what a partner can demand of her.
With more detail, you can dig deeper and find mirrors that are subtler or more complex. Perhaps "right-guy-wrong-time" means your protagonist has, perhaps unconsciously, a conceptual "life plan," a sense of what she expects to happen when - and this guy is coming in at the wrong step. With that view, a possible mirror is another guy who does fit the current step - but he's got the potential the disrupt "the plan" entirely; send the protagonist's life on an entirely different and less certain path. Here, the tension is your protagonist's life expectations clashing with reality, with enticing opportunity, and possibly with grave risk to her other hopes and aspirations.
These are a few examples, and I'm sure you can spin up many more. I hope they demonstrate how you can play around with mirroring to find the complement to your love triangle! I've tried to stress how the detail you stress with your mirroring is intimately tied to the central mood, tension, and conflict you want for the story or subplot.
If you want real rivals, why make one of them "wrong"? As an exercise, why not try to make both of the romantic rivals "Mister Right", just in different ways? For example, one of the more interesting aspects of the early 90s Winona Ryder / Janeane Garofalo romcom "Reality Bites" is that a good case could be made that the corporate guy (played by Ben Stiller) was a way, way better fit for Ryder's character than the douchey artist (Ethan Hawke). I think the movie tried way too hard to make the audience root for the latter instead of the former but that sense is still there.
I think the best romantic comedies - Annie Hall, 500 Days of Summer, Celeste and James Forever, and Silver Linings Playbook for 4 examples - portray people interacting with each other, sometimes finding love, sometimes falling out of it, but always acting human. A guy who is nothing but suave and awesome and "Mister Right" all the way through is around 18 times more boring than a guy who has strengths and who you find likable but who also real flaws beyond "OH HE'S SUCH A MAAAAN" or "hey, he sure is messy!!! He accidentally put his tie in the refrigerator!!!!".
High Fidelity, the book or the movie, is another fantastic example. There you have your two main characters who aren't stuck in a "right place at the wrong time situation", they've tried to make love work between them and they've mostly failed at it. The story that's told is as much a slow epiphany by Rob (the John Cusack character) that he's kind of a jerk as anything else. His "rival" for some of the story, the guy played by Tim Robbins in the movie, isn't really that bad of a person either, he just happens to be the guy who is currently dating Rob's ex and that alone makes him the villain in Rob's story. Rob has some real hang-ups that cause him to do some really, really bad things to his GF (whose name completely escapes me at the moment, sorry), and not all of those hang-ups are resolved by the end. We feel for him, though, because he's making a concerted effort to fix the way he thinks about things.
You already have a rival: Time. Or Circumstance, or whatever your plot complication is which keeps the lovers apart because it's the Wrong Time.
Isn't it even more tragic/angsty/yearning that there's no third person keeping the two apart, just the adult recognition that the couple simply can't work out at this moment in time? That it's the choice of one or both parties that Something Else is more important than the relationship? And then the spend the rest of the arc trying to get back to one another and make it the Right Time.
Maybe someone who seems to be the exact opposite of one of your leads? Which weaknesses do they have?
If one of them is sloppy with no real goal in life why not introduce someone who is neat, driven and successful? (or at least seems to be..) Later on you could reveal the rivals weaknesses which are even worse then the leads. Maybe you could turn one of his/hers perceived strengths into a weaknesses by overdoing it? Or reveal him/her to be a fraud?
Bonus points if the characters appears after the couple fought over one of his/her weaknesses.
Just a thought.