Stocking 45 gallon

Hello, I've managed to source a 45-gallon tank to be the final tank on my breeding rack (others are 35, so this is bottom tank). I wasn't originally aiming for a tank like this, however, I came across this tank, with tight fitting lid and working lights for a cheap price and grabbed it while I could.

The intention of this breeding rack is one tank to be apistos, one tank to be rams. They're 35 gallons each, I have plenty of grow out tanks as well as these tanks will be divided as well.

The original intention, back when I thought I was getting a 30 or 35 gallon, was shell dwellers. I am still interested in this, but with it being a larger tank, I am also curious as to what else would be possible within this tank.

I currently have a 30-gallon long nano community, a 10-gallon block Black Tiger Dario/Badis tank, an 80-gallon high live-bearer pond with additional shrimp, various danios and wcm/gwcm. I also have a 35-gallon long with an axolotl called Fish, Son of Odin. (I also have tanks that have a breeding quad of lemon bristlenoses + two younger and a young quad of super reds.)
So I'm interested in something that is different to what you'd find in a "common" lfs. I have also got another 45 gallon that is cracked, cracked during transport, that I will be turning into a crab tank using a spare 30ltr to be the water container.

Measurements:
Inches - 39x15.5x17.9
cm - 99x39x45

My tap is hard water, about 10-12dgh, but seems to be getting harder with a recent test coming out at 16dgh. However, I also use RO for my soft water community and the apisto and ram breeding tanks. These are RO with discus minerals to make sure that the minerals are at a better reading.

Part of the reason I moved onto RO was that my water supplier has likely changed one of the chemicals involved and the nitrate also increased from under 5 to over 50. I lost all my dwarf cichlids and most of many of my tanks after doing some tap water changes (conditioned).

I am curious as to what others would do in a tank this size and dimensions. It also has a nice fitted lid, my only tank that has a nice fitting lid. I am interested in trying larger fish; I never really got the opportunity to with the size tanks I have.

I want to try something new, something that I have not done before. And for that, I am not sure.
I could always do the shell dwellers, possibly with some julies, and make it a colourful semi-aggressive community. Or something else. (I don't mind doing species-only tanks).

Open to any ideas.