The phrase "could be dangerous" is correct and can be used in written English. This phrase is often used to suggest that something may have the potential to cause harm. For example: The chemicals in the lab could be dangerous, so make sure you wear safety goggles.
This could be dangerous.
Everything could be dangerous.
"He could be dangerous.
"It could be dangerous," Ms. Popal said.
"It could be dangerous on the stage".
Simon said statistics could be dangerous.
That could be dangerous for the proselytizers.
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MA of Applied Linguistic, Maquarie University, Australia