Current Datamines: (Updated 2024-06-20)

This information is based on data found in MY HERO ULTRA RUMBLE itself. Remember, nothing here is confirmation of anything releasing sooner or later, and is subject to change at any time.
So far datamines from UltraRumble.com have been consistently correct with releases and nothing has been wrong. Things found in the past in previous seasons have in fact released in the following seasons.
(Eg. Twice and All for One found prior in Season 2, released in Season 3/4. Mirio and Dabi's Quirk Set found in Season 4, released/releasing in Season 5.)

Be aware, the image of the unreleased character emblem icons with release dates that is being shown around is NOT a datamine. This image is purely someone's guesses. There is no data to suggest these release times in the image are correct.

New Game Mode Data
Code Name: SQUAD_KINGS_BATTLE
Game Mode Name: LEADERS BATTLE
Description: 仲間と連携し\nリーダーを守り抜け！    -    Work with your friends to protect your leader!
There are JSON data files as well for this mode similarly to the existing modes that are playable in game. As of 2024-03-27, this data was removed.
Overhaul Data
Role/Type: Unknown
Default Quirk Skills unknown at this time.
Name Text Data Found: m_character_name__34    -    Overhaul
Emblem Text Data Found: Overhaul Beginner, Overhaul Pro, Overhaul Ace
This is all known Data and Assets known for this character. As of 2024-04-10, this data was removed.
Shoto Todoroki Quirk Set Data
→
Quirk Skill Set unknown at this time.
This is all known Data and Assets known for this character quirk set. As of 2024-04-10, this data was removed.
Denki Kaminari Quirk Set Data
Role/Type Change currently unknown.
Quirk Skill Variant Name: ライトニング -> Lightning
Quirk Skill Variant Description: 上鳴電気の"個性"技セット ライトニング -> Kaminari Denki's "Individuality" technique set Lightning
"available_at:": 2024-05-22 13:00:00 JST This is NOT the same as a Release Date. (This has been removed since 5/22/2024)
Variant Quirk Skills unknown at this time.
This is all known Data and Assets known for this character quirk set.
Himiko Toga Quirk Set Data
Added in the data on 5/22/2024.
Role/Type Change currently unknown.
Variant Quirk Skills unknown at this time.
Data entries have been found for the Himiko Toga quirk skill set where the "character_magazine" is, which holds data for Quirk Skill level data like ammo count, reload time, etc.
Click here for a more detailed breakdown with images
This is all known Data and Assets known for this character quirk set.
Eijiro Kirishima Quirk Set Data
Quirk Set Name & Skill Unknown at this time.
As of 2024/06/19, in the assets for Projectile assets/data, there are sub folders for certain characters called "Var01". "Var01" folders and naming is generally used for character quirk sets, and looking at the characters with "Var01" sub folders, all are characters with released quirk sets and a couple of unreleased quirk sets. Characters with the Var01 folder include Izuku Midoriya (Ch001), Katsuki Bakugo (Ch002), Tomura Shigaraki (Ch015), and Dabi (Ch017). The ones that also have "Var01" sub folders are Eijiro Kirishima (Ch008) and Himiko Toga (Ch018). So only characters with quirk sets have this "Var01" folder, it is safe to assume Eijiro Kirishima and Himiko Toga are having quirk sets in development (Also Toga who we previously knew had a qurik set in development, has the previous datamined information above that also still exists in the masterdata). When will these quirk sets release? It is unknown at this time.
Click here to view a more detailed post about the Eijiro Kirishima qurik set here
This is all known Data and Assets known for this character quirk set.