On the 18th of June at 3:00am UTC, the in-game notifications revealed a new pair of Summer-themed Heroes as silhouettes.
One’s a beefy beefcake wielding a shovel/large spade, while the other is rather slender with quite long hair. That narrows it down to about half the FE cast, then.
The banner itself will be available to summon from on the 20th of June, 7:00am UTC. You can expect a trailer for the banner to pop up sometime soon.