You can use a Slot as the target, just as easily as a character. If you want to have your CC effect, or AoE effect on the same key for multiple teams, then this is a decent approach.

If you go with a 10 step mapped key, then five of the presses will do nothing. I probably would not go this route, myself.

You can also create a Key Map for each of your character sets. Go with something like: General, FTL DPS and Control for all teams/character sets. But then also go with a Mage Keymap and a Warlock Keymap. Then assign the Keymap to a given team, so when you load that team, you get their Keymap and all of the mapped keys contained therein, in addition to the Keymaps that apply to all teams. I've got a Keymap specific to a few of my teams.