What are some techniques to make your acoustic "strumming" more dense (containing distinct different tones)??
Yeah play with your fingers and be sure to pluck strings at varying strengths, depending on parts that need to be emphasized, emotion, etc. Don't just play like you are playing a GP5 file, where all notes have the same emphasis.
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Try changing strumming position (nearer to or further from the bridge) to achieve different timbres.

Failing that, buy a 12 string!
Use open strings where ever you can.
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When you say dense, I automatically think bigger gauge strings. You might want to look into that.

Otherwise Use open strings when wanting to spice up finger picking.

Having complex strumming patterns is also fun.