I'm an industrial software engineer... I regularly use high end industrial controllers, mostly Allen Bradley. Anyway, I _know_ what controllers are capable of.
Here's my list
1) A real programming interface. The wizards and easy setup options are great for the newbee's, but I'd really like to be able to write simple logic... if, then, else, named variables, Booleans tied to inputs and outputs, simple mathematics and logical operators, etc. Some sort of fully functional scripting language. Yes, I realize that many would not make use of such a thing, and I don't have a problem with keeping the easy stuff up front, but access to the code for those that want to use it would be great.
2) A decent relay input/output block. Apex has their breakout box, which is a horrible design, by the way, but it does give you some basic binary inputs. The only outputs are through 120VAC controlled outlets.... Why not a bank of relay based dry contact outputs? Being able to trigger a DC powered device via controller output seems a pretty basic need. The only similar function on Apex is to buy their solenoid, and wire it to the FMM or EB832. Expensive, and inflexible.
3) A decent HMI. Apex seems to be moving away from this, mostly due to the availability of inexpensive tablets. IMHO, these are _not_ satisfactory. A waterproof, AC powered, always on display, easily mounted near the aquarium, with customizable display options would be incredibly valuable. Mounting a cheap tablet is _NOT_ a sufficient replacement.
4) Here's a simple one... a link from Fusion to the Local 'classic' page. Apex is already sending info to Fusion... sending it's local IP would be trivial. I know, it'd only work if you were on the same local network... but I think we can safely assume that most controller configuration is done while local to the aquarium. If you're not local, well, it isn't going to work... but it sure would be handy for those of us who's IP's are always changing due to DHCP configuration on our routers.