Depending on the use of the illusion, you may need to look to other rules to resolve the action. For example, if instead of a fire the characters created a vision of an unspeakable horror, you might reference the rules for temporary madness. Try to encourage the players out-of-the-box ideas so they can find a creative solution, and then apply the existing rules to resolve the situation.

Sometimes ideas that sound great have a low likelihood of success. As the DM you can set the challenge rating on a related skill check as part of a multi-step plan. I recommend you be clear with your players about what they need to do (roll) to succeed and the consequences of failure, so they are clear on what the stakes are.