I'm gonna say this and it's not what FOWLR people want to hear, but it's my 2 cents.
1) FOWLR is so, so, so ugly. Something about plain rock just looks like an entire tank is being wasted, it literally detracts from the fish
to me. Put in some basic soft corals and suddenly it looks like a great system. My ideal "FOWLR" would be Photosynthetic Gorgonians, sun corals, secluded rock structures of paly/zoa's, etc etc.
These corals can be kept with bare minimum lighting and do okay -- It will give your tank a little pop that's missing. They won't need to be the fanciest or crazy looking corals. They won't need much as they're being fed heavily by fish waste and bits of food. And they require very little to no lighting. And lastly, they cost very little, and have almost zero maintenance.
To me, that's a win, win, win.
2) FW fish are better for the big aggressive fish thing. Yep. I said it.
Gar are cool
$400,000 platinum Arowana (not a typo)
African Tiger Fish
Freshwater Barracuda
Golden Dorado (yes, people keep these big guys!)
I know, I'm a broken record. I like reefs, and big aggressive FW fish.
If you FOWLR guys feel daring, and want to invest in a BIG tank (i'm talking 1500g+) you will be heavily rewarded by having fish like these. They're amazing creatures, more simple in pattern but equally colorful, and typically easier to setup an Aquarium Quality biotope.
Most of these big guys are river fish, setup a LONG and WIDE tank and put a ton of flow on one end, and some brush on the other end. These guys go great with dither fish, and look amazing with a coarse sandy substrate and a few river rocks. you can have some vegetation with these guys as well.