I was thinking of the Octopus BH-1000 as a HOB skimmer for the 56, rated for tanks up to 100g. It has an external pump so nothing in-tank, also surface skims. I've reviewed good things about it.
I would initially keep the HOB fuge but only until I get an under tank fuge. I measured the stand and there would be room enough for a standard 15g.
I saw this guys video who made a DIY in-tank over flow. Pretty cool and looks like something I could do. Without drilling tank
Anyways those are just a few ideas. Most likely to change...who knows?