The characters listed are available in the any banner.
Aizawa or Shoto is a must-have if you want the best out of your rerolls, and at least 2 other characters.
When there is an Impact Fest x1.5 rates banner, if there is only one Impact Fest character, reroll for Aizawa or Shoto with any one additional character. If there are two Impact Fest characters, you can skip the additional character reroll character in order to prioritize 2 Impact Fest units with Aizawa or Shoto.
Additionally, you may want to prioritize your reroll units based on the tag of the Impact Fest unit, such as Class 1-A Student, Pro Hero, Villain, etc.
Do bear in mind that this list may change, but this is very unlikely to happen. Credits to SDL and Ludi for this list.