This was something I never understood about FFXIV's design moving forward... Aion did a similar concept by creating combos that were either trees or simple 1-2-3's into one hotbar but this allowed them to branch out things and make more elaborate skills and rotations.

One idea I want to see FFXIV mimic is the "charge skill" thing they did too.

https://youtu.be/MFRCxc8-jSc?t=67

The concept was pretty simple. The GCD would continue from 1-2-3 time depending on how long you decided to hold the skill for. One press and you'd begin charging the skill, hitting it again and it would be used at the level you left it at. tapping it would just give the level 1 of it, pressing it again after a duration of one GCD would present it at a level 2 charge, and then pressing it after letting it go ANOTHER GCD would be the maximum. This allowed for a variety of options because not only did this mean you could invest skills in damage for burst or decide on just outright using skills for dps.... But different charges sometimes offered different abilities. One example was how a skill could change from a single target into an aoe going from charge 2 to 3, so you had reasons to actually use in between charges. Or another was changing the status ailment done by a charge, level 1 being a quick bind, level 2 being a slow, level 3 being an aoe sleep/silence. Each class had 2-3 of these but weren't necessarily considered a "staple" of the classes on every single class, just another thing you COULD do in certain circumstances. Mages for example typically wanted to charge everything to the max like a cast normally, but some physical classes may lose range if the target is running or maybe it just wasn't useful 100% of the time and limited to specific scenarios.