Hi. I think the words "that showcased" should be used here but I am not sure why the word "showcasing" can't (or shouldn't?) be used (if it indeed can't be used).
It was clear that this was a great event showcasing great ideas.
It was clear that this was a great event that showcased great ideas.
Top answer
'Showcasing' has no time or tense element in your first sentence. Both sentences are fine.
— Mister Micawber
'Showcasing' has no time or tense element in your first sentence.
Both sentences are fine.
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