At least one Modifier will always apply when spending Threads. Spending more Threads improves the chances of multiple Modifiers applying.
Expedition Modifiers which can be applied by Threads:

Expedition Encounter in area will be massively larger.

All Runic Monsters in Expedition Encounter will be at least Rare

A Runic Obelisk will spawn as part of the Expedition Encounter, which can revive all Rare and Magic Expedition Monsters when activated.

Expedition Encounter will contain at least one unearthable chest which will drop a Logbook.

Expedition Encounter will contain Runic markers for Runic Lieutenants, powerful monsters which drop extra reroll currencies when slain.

Remnants in Expedition Encounter will have additional upside affixes.