u/YunieSayaka

Some context: For the past 4-5 years, FEH has always put at least one unit that got a PRF skill within a certain timeframe (July New Heroes to December Legendary banner) on their summer banner, starting with Summer Freyja. It's safe to assume that, since this trend has been going on for so long, they'll continue this. Seeing as there are two summer banners this year, that means at least 2 of the listed characters in the above tier are getting a summer alt, and this is what the first tier list is about. 9

The second tier list is based on a more recent trend. Over the last three years, IS has "revised" one old Legendary unit and given it a summer alt. In Book 7 it was Ephriam, in Book 8 it was Hrid, and in Book 9 it was Celica. Considering they're running low on units they can use from Book 2 due to recent alts + summer alts, I included all units currently with a Remix just to give people hope.

Note that these rankings are flawed in that higher tiers =/= more likely to be on a Summer banner, but I would bet money that at least 2 characters from the first tier list and 1 character from the second tier list will show up on the summer banners.