I'm not very creative, so I don't really have any ideas myself. Although a heavy weapon shrapnel launcher like those used by Fallen Captains might be cool...
Anyway, I would recommend going to the Creations page in the Community section and using the filter 'Rating'. Many of the top hits are fan concept art of exotics.
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Lord of wolves
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It's not heavy but its the shrapnel launcher
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It would be cool if there was a more powerful heavy version of it.
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