No collaborations that could affect the main timeline, or do not work time wise.
If Alice is at the beginning of the planet, and Bob is at the end, they cannot logically meet. You can circumvent this by having them pulled out of the timeline by a higher being or god, as long as it follows the below rule.
Be careful with higher beings, gods, and etc.
Your series can use magic, higher beings, deities, gods, etc. as long as it does not discredit other series'. Your character can absolutely meet some higher being. Your character cannot meet "the creator of everything" and still be considered canon. It is assumed that every type of magical thing is possible, and that no specific thing created all of it. Try to avoid "creators" in general. However, higher beings such as "god of fire" are okay, as it is assumed they are the same being given different names throughout the long time of the timeline.
Avoid involving characters from other series outside the timeline
It doesn't make sense that Alice would meet Charlie if Charlie never exists in this timeline. Non-canon easter eggs and references are okay, but not canon inclusions.
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