Thoughts on how to improve tank (aka the next move)

Hello,

It's been a while since I've posted on here. I have a nice, established (1 year or so) 55gal FOWLR. White sand bed, 60lbs LR, 4 Nassarius Snails, 1 Chocolate Chip Starfish, 1 Coral Hogfish, 1 Tomato Clown, 1 Skunk Clown. Equipment: Remora AquaC Skimmer w/ Mag3 pump, 300W heater, 2 Koralia 3s, 1 other jet.

So I have a nice tank, in my opinion, but I have had poor luck keeping many fish other than what I have now which I have had for a year. The tank always seems a little barren to me so I am thinking about expanding/improving. So what's the next move? Sump, cannister, convert to reef, something I haven't even thought about to improve water condition? I would like to hear your thoughts.

Seriously. Anything you would like to share I would like to hear. Usually an opinionated bunch and that's all I'm looking for, some personal opinions.

Thanks for taking an interest. I do keep it clean. I let it go a few times and only cleaned the front and felt that instead of looking "natural" it just looked filthy so I started keeping it clean all around.

I've lost, in my current tank setup, 5 chromis introduced as a group, the "mate" to my skunk clown, a lawnmower blenny, and worst of all a gorgeous niger trigger.

The chromis died off pretty quick I think from poor initial health. One lasted a few months but then died out of the blue.
The one skunk clown was healthy but I think was slowly weeded out in territorial disputes.
The lawnmower blenny starved to death (which is terrible). I couldn't get him to eat any supplemental food like Nori, as there is not much hair algea in the tank.
The trigger was my pride and joy. Perfectly healthy for 4 months. Unfortunately, I had to leave for a two month period for work and when I returned there was no sign of him. I had a knowledgeable guy watching the tank in my absence. He said all was fine so it's anybodies guess.
No signs of disease in any of these cases.

The tank just always looks plain to me. I would like to have more fish. A schooling group or some more diversity. A reef tank interests me, but if I can't keep certain fish at the present time I would hate to invest any more $$ unless I am confident with how things are now. If that makes sense.

I could put a sump in the stand, but is that worthwhile for me?