So, for someone who uses very little addons and barely any in terms of combat addons, an Issue I often have is noticing when enemies within a large group is casting something that can be interrupted.
So it would be really great with an option, so that Target Frames that are casting an ability you’re able to interrupt, will be brought to the top of the Pile, so you can easily see it, and maybe an option to further increase its size slightly while its casting to help it stand out even more.
A few things you can do to make interrupting easier in the default game.
1: Make a macro that sets your mouseover to your focus target and, optionally, set it with a mark like diamond.
2: Keybind that macro to something intuitive and comfortable for your mouse hand, mine’s on scroll down.
3: Use focus interrupt with a mouseover override as your default interrupt.
4: Move your focus cast bar away from the focus frame to somewhere more central and clearly visible. Mine is above my cooldown manager.
This would be great, however for now you can also use tab target as it will automatically prioritise something casting an interruptable spell.
If you’re willing to look into addons.
1: Improve your nameplates. Something as simple as mobs with mana having blue health bars or changing the bar dimensions, clickable area, and spread amount makes it a lot easier to both see during combat and pick an appropriate focus target beforehand.
Oh cool, I wasn’t aware of this feature actually, very helpful, thanks 
Now I just need to be able to see when they’re actually casting 
Download the addon called Targeted spells. it will have visable bars with casts from nearby enemies, you can even make it call out the spells being cast by voice.