After you have selected your team in the character selection menu, another "Tactics Menu" appears. Here you see all five characters with their abilities in a column next to them.
Next to the abilities is a simple 'arrow up or down' This sets the order for which they will use their abilities.
The order is 'top to bottom' so there isn't even a need for numbers (it's spatial and visual, less clutter on screen)
I am very confident this adjustment would make your game vastly more enjoyable.
There are other options that have been suggested like: 'Second auto-attack default mode' 'have character only use X when Y occurs' and you could get very deep with my above suggestion by adding a team queue (all the abilities in one queue) or sliders for abilities with match parameters...
Ex: Leia uses stealth when below: [25%] [50%] [75%] health
Ex: FOTP uses 'Gun Down': [default] [only with advantage]