Well this idea is probably not the best place to come from from what I have seen. A monogamous mindset doesn't work in poly. It seems to work better to come at it from an independent person point of view. You are your own person, your partner is their own person, your other partner is their own person.
What couples seem to miss is that this is THREE people. Not a couple and their unicorn... sure that is the beginning sexy NRE stage, but for the long haul, if you truly want the long haul... its okay if you don't, then I would suggest breaking your thoughts down into you, her and her... you and her, her and you and her and her.... not couple and her. YOU as an individual have a lot to offer, and aren't you fortunate that you are with someone that is wonderful and also has a lot to offer....
Secondly, this is VERY NEW, give it some time and just enjoy your partners excitement...just because your wife is bi doesn't mean that you will end up in a triad... it isn't the most common dynamic. She may very well want her own lover one day without you. In fact, that is most likely to happen along the line. Even after a triad unicorn thing happens.... the two of them might want to be together and you aren't invited any more... or you and the new woman might want to be together and she isn't invited any more, or the two women might not want each other any move... making a vee.
I suggest you do some tag searching on here for "unicorns," "secondary" and "triads"....and "vees" (have I not mentioned this on this thread already!
)It sounds like you are new to poly and have some educating to do.... glad you are having a good time with your partner, but before adding another, I suggest you make sure you know what to expect and what to look out for. That way no one gets hurt.