A possible plot could be that Morgana and Merlin's kiss is a result of a love potion gone wrong. Someone in Camelot, who wants to cause chaos, slips a love potion into their drinks. But instead of just a simple love - stricken state, the potion makes them realize their underlying feelings for each other that they have been suppressing. Another idea is that the kiss occurs during a time - traveling adventure. They are sent back in time to a period when Camelot was first founded, and in that different time, without the baggage of their present - day enmity, they fall in love and share a kiss. It could also be that the kiss is a form of magic exchange. Merlin's powerful magic and Morgana's unique brand of magic combine when they kiss, and this new combined magic could be the key to solving a major problem in Camelot, like a drought or a plague.
One idea could be that their kiss is a moment of realization. Maybe they've been enemies or at odds for a long time, and the kiss makes them see each other in a new light. For example, Morgana realizes that Merlin's magic isn't as bad as she thought and that he has been trying to protect Camelot all along. Another plot could involve a magical bond that forms through the kiss. This bond could give them shared powers or make them telepathically linked, leading to some really interesting interactions as they learn to deal with this new connection. A third idea is that the kiss is a secret pact. They kiss to seal a deal to work together against a greater evil that threatens Camelot, but they have to keep their new alliance a secret from everyone else in the kingdom.
Well, perhaps the kiss is a magical accident. Merlin was trying to use his magic to save Morgana from a spell, and in the process, their lips touch, creating an unexpected spark. This could lead to a series of events where they both start having strange dreams about each other and their fates seem intertwined. Or, the kiss could be a part of a prophecy. In the old texts, it was foretold that a kiss between Morgana and Merlin would either save or doom Camelot. So, they are forced to explore their feelings for each other while also trying to figure out how to fulfill or avoid the prophecy. And, it could be that the kiss is a moment of pure passion in the middle of a battle. They are both so caught up in the heat of the moment, fighting side by side, that they just kiss spontaneously, and then have to deal with the aftermath of their actions.