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I agree with Matt. Add at least 5 more cherry barbs. You could probably add a few dwarf cichlids if you are into them. Another option is Gourami's or a some Angels.
personally i wouldn't go rainbows since they like much warmer water vs. the corys and danios you have. it is around a 5-10 degree difference. also smaller fish like danios are more timid around larger fish like rainbows. gouramis are a better option. with a 75g you could do pretty much any gourami besides giant gouramis.