Double-clicking a monster should automatically mark it.

As a tank I hate having to have all marks taking up space in my hotbar. It's also pretty annoying to have to either waste a keybind on marks or waste time individually clicking the monsters then the mark on my hotbar.

Double-clicking to mark a mob should be implemented. Both on the mob itself and on the enmity list. There is already a system in place to see which mob is 'next' (Letters next to their names). In addition, double clicking the next mob (In alphabetical order) on your enmity list should automatically mark that mob as '2', and '3', and so on.

I feel like this would make tank's lives a little easier.

The only mark I have is 1. I can't speak for WAR since I only have PLD leveled, but I can't think of a single fight outside of maybe moogle where marking mobs is necessary outside of a single one. As a PLD I can easily keep threat on as many mobs as are around and just need to mark the 1 I want dmg focused on. Aoe threat is pretty easy for PLD.