Depending on how the Max Soup worked, it may give Pokémon from Past Gens the ability to Gigantamax.
Ignoring the entries for any species that is capable of Gigantamaxing, let's take a look at interesting entries,
Specifically, balls that cannot be used in Sword and Shield.
Species
Location
Notes
Sports Ball Caterpie
HGSS National Park: Bug-Catching Contest
To be evolved to Butterfree
Sports Ball Metapod
HGSS National Park: Bug-Catching Contest
To be evolved to Butterfree
Sports Ball Butterfree
HGSS National Park: Bug-Catching Contest
Safari Ball Pikachu
RSE Safari Zone
Cherish Ball Pikachu
Any event that has Cherish Ball
Safari Ball Machop
HGSS Safari Zone: Morning/Day
To be evolved to Machamp
Safari Ball Machoke
HGSS Safari Zone
To be evolved to Machamp
Safari Ball Machamp
Evolved from Machoke
Cherish Ball Machamp
Any event that has Cherish Ball
Safari Ball Gastly
HGSS Safari Zone: Night
To be evolved to Gengar
Safari Ball Haunter
HGSS Safari Zone: Night
To be evolved to Gengar
Safari Ball Gengar
Evolved from Haunter
Cherish Ball Gengar
Any event that has Cherish Ball
Safari Ball Krabby
HGSS Safari Zone
To be evolved to Kingler
Cherish Ball Krabby
Any event that has Cherish Ball
To be evolved to Kingler
Safari Ball Kingler
Evolved from Krabbly
Safari Ball Lapras
HGSS Safari Zone - Surfing
Cherish Ball Eevee
Any event that has Cherish Ball
Cherish Ball Snorlax
Any event that has Cherish Ball
Cherish Ball Munchlax
Any event that has Cherish Ball
To be evolved to Snorlax
(Also for non-Cherish Ball entries, it'll be fun to breed the ball and HA onto them )
Additionally, if Gigantamax Melmetal continues to be not distributed by any raids (events or otherwise),
this may be the only way to get Gigantamax Melmetal. (By feeding a Melmetal from PGO the Max Soup)