What if I fall for my metamour?
It happens. The polycule can absorb it if everyone involved consents to the change; what it can't absorb is the change being acted on without honest conversation. Talk to all three people involved — your partner, your metamour, and yourself — and don't act before there is clarity.
Developing feelings for a metamour is one of the predictable possibilities of a polycule with cross-paths. People you spend time with, who share important context with you (you both love the same person), can become emotionally significant on their own. The polycule structure can accommodate the development if it is handled with the same honesty and consent the rest of the structure requires.
The first move is not to act. Notice the feeling. Sit with it. Don't disclose it to the metamour without thinking. Don't initiate anything physical or romantic on the strength of a few weeks of feeling. The feeling may settle on its own; it may grow; it may turn out to be a different feeling than you initially read it as.
If the feeling persists and seems real, talk to your shared partner. Not the metamour first — your shared partner. They have a stake in any change that involves the metamour, and they should know what is in motion before the metamour does. The conversation is hard: 'I think I am developing feelings for X' is information they may have complex feelings about, and you should give them space to have those feelings before pushing forward.
If your shared partner is supportive and you want to proceed, the conversation with the metamour comes next. They have full agency to say no, yes, maybe, or not yet. They may have feelings of their own that you do not know about. The healthier version of this conversation does not assume reciprocation and does not treat the metamour as a prize.
What can result. Sometimes the structure absorbs the change beautifully — the network becomes a triad or a denser polycule with stable bonds across more edges. Sometimes one of the involved people is not okay with the change and the development does not proceed; the existing relationships continue. Sometimes the development causes lasting strain even when initially welcomed. All of these are real possibilities; the only one that is not okay is acting without informed consent from everyone affected.