FFXIV's macro system is very diverse and useful, but there is still room for improvement! I have three suggestions that I hope can be used to make a good system even better.

Item macros
This one is the most unusual shortcoming. There seems to be no way to use an item inside of a macro. Sure, you can assign the item to a hotbar button, but it's generally a waste to use up a valuable hotbar slot for something with such limited utility. I want to be able to incorporate items into other actions - for example, a macro that will let me use a Job Ability if it's available, and down an appropriate potion otherwise:

/micon "Fight or Flight"
/action "Fight or Flight" <me>
/item "Hi-Potion of Strength" <me>

This macro would use Fight or Flight if the timer is up, and if not give me strength boost via potion instead.

Mount macros
When I'm in town, I want to Sprint. When I'm in the field, I want to ride my mount. Right now, I have to waste two hotbar slots to do this, when I'd prefer to incorporate them into one macro:

/micon "Sprint"
/mount "Company Chocobo" <me>
/action "Sprint" <me>

Please add a macro command that allows you to select and use a mount of your choice!

More micons
When building my user macros, there's a surprising number of things for which the /micon command doesn't work. Crafting icons are particularly hard-hit by this; I have several user macros that I use for crafting, and I'd like /micons for Inner Quiet, Basic Synthesis, and Basic Touch, among others. Take Inner Quiet, for example:

/micon "Inner Quiet"
/action "Rumination" <me>
/action "Inner Quiet" <me>

This allows me to use Inner Quiet and Rumination on one button, saving hotbar space - but like all Synthesis user macros, /micon doesn't work.

While I understand that crafting icons are complicated by the fact that the same ability has a different icon for different crafting classes (for example, the Basic Synthesis icon for Culinarian shows a frying pan while the same icon for Weaver shows a needle), I think that an micon should be available for any type of icon in the game, not just for a limited selection of DoW and DoM icons. If a Leatherworker wants their Tricks of the Trade hotbar button to show the icon from a Unicorn mount, they should be able to do so!