It is too early to decide top threats in this OL+UL, but these are a few Pokemon that I think will be top threats
Thundurus-I
Types of Underlings:
Possible Electric Underlings: Zapdos, Rotom Forms, Magneton, Ampharos-Mega, Galvantula, Heliolisk, Eelektross, Jolteon, Lanturn
Possible Flying Underlings: Aerodactyl-Mega, Salamence, Gligar, Crobat, Mandibuzz, Braviary, Tornadus, Gyarados, Noivern, Yanmega, Honchkrow, Golbat, Vivillon
Viability: Electric and Flying are amazing types that cover each others weaknesses. They both are very fast and very offensive.It could create very dangerous VoltTurn Cores. Magneton is a great underling, as it covers for almost all of Thundurus and other flying types. Rotom-Heat is another good Underling as it bulky and covers the ice and ground weaknesses that other teammates struggle with.
Charizard-X
Types of Underlings:
Possible Fire Underlings: Entei, Infernape, Rotom-Heat, Chandelure, Darmanitan, Arcanine, Fletchinder, Emboar, Ninetales
Possible Flying Underlings: Aerodactyl, Salamence, Gligar, Crobat, Mandibuzz, Braviary, Tornadus, Gyarados, Noivern, Yanmega, Honchkrow, Golbat, Vivillon
Viability: Charizard-X will be one of the top Overlords in the tier. It has two very offensive types to pull from, and it is already S-Rank in regular OU. The Flying Type underlings will be able to be very strong thanks to Charizard having Fire Type Underlings to counter Ice types such as weavile and kyurem-black.(If sun is allowed, Ninetales can be run to run sun offense.) Speaking of Kyurem-Black...
Kyurem-Black
Types of Underlings:
Possible Ice Underlings: Abomasnow-Mega, Glalie-Mega, Mamoswine, Cloyster, Rotom-Frost, Sneasel
Possible Dragon Underlings: Salamence, Hydreigon, Goodra, Kingdra, Haxorus, Noivern, Flygon, Dragalge, Tyrantrum
Viability: Kyurem-Black has the highest attack out of all non-mega pokemon in OU and below (Only beaten by Mega-Chomp and Mega-Heracross). With this and other really powerful underlings such as Salamence, Mega Glalie, Mamoswine, and Noivern, Kyurem-Black will be a top threat in the metagame. It can even run hail with mega Abomasnow to boost damage even more
It is likely that offense will be more common than stall, as most bulky Overlords either have a single typing that makes them countered by many things (Chansey, Mew, Hippowdon) or get bodied by one of the three pokemon i talked about above (Skarmory, Slowbro, Ferrothorn). I think that Dragon or Flying type overlords will be extremely good in this metagame due to the sheer power and good underlings that they have at their disposal.