The biggest/most important would be to have a temperature probe. This will give you a failsafe in case your heater gets stuck on so you won't fry your tank.
Next thing to add would be a float switch to control your ATO. You can do this separately, but incorporating it with your controller will streamline everything and you can also program it to turn off your ato pump when you're doing a water change. You can add lots of different switches like one to tell you when your ATO container is getting low if you wanted, or you can add one that tells you there's been a water spill/overflow.
Anything else would be dependent on what you're running in your system, so a PH monitor would be nice (they're also fairly cheap at around 30 dollars) but isn't a necessity, it definitely would be something to get if you were dosing kalk, however. Also if you're running a vortech the apex is able to control that as well. Since you have an HQI I'm guessing you are going to stick with that. If you do, however, switch to LEDs you can control those with a controller as well and have ramp up and down functions and storms if you'd like.
Some things will just be preference, like being able to see your tank info and control it through an iphone/remote computer with the apex is nice for me but may not be a necessity to you. You can connect the reefkeeper to the internet but it requires a separate, expensive module.