Yeah, as mentioned, I'd likewise prefer only occasional visits/team ups, but nothing that's permanent. A good contact for the guys when they need an extra hand. Otherwise, she's typically living outside of NYC somewhere, maybe even further away, just trying to quietly exist. That way she can be around, but not annoyingly so.
And, please, no need for her to
have to be someone's love interest/girlfriend... No harm in one or more of the guys having a little short term interest, or in one of them vaguely tweaking her interest a little on occasion but not necessarily enough.
It would be refreshing to skip the old need to pair a character like her up and instead give her a viewpoint that runs more along the lines of,
"We're mutants, possibly hybrids, and the only ones of our kind so there isn't much point... plus, no offense, I love you guys, but you all kind of annoy me in your own ways." 
Being fellow mutant Turtles doesn't inevitably mean any of them will managed to be the guy for her.
If the guys at times can find an interspecies love interests they're happy being with, then so can she if she so chooses.