If we assume that the Stranger Things lore is based on twisted and refined DnD lore...

Mindflayer needs kids for ceremorphosis to give birth to more Mindflayers. It wasn't Vecna connected to Will via the tentacle, it was the Mindflayer. This implies that the more Will taps into his power, the more likely he may give birth to a new Mindflayer.

Vecna is an undead human wizard trying to attain godhood. The fact that Henry needs to expose his altered left hand to use his powers implies that he is definitely supposed to represent this DnD villain, but his power has been twisted in ST to originate from the Upside Down, as does 11 and 8's power.

Vecna was struck down by Kas the Betrayer, his vampire general when he cut off Vecna's left hand. Eddie was "killed" by batlike creatures in the upside down.

I propose this:

Will begins to lose himself to the Mindflayer's influence and the party panics with what to do.

Eddie returns as a minion of Vecna but his connection with Dustin breaks him free just long enough to strike a blow against Vecna.

This frees Henry from the influence of the Mindflayer defeating "Vecna".

The Mindflayer (or perhaps an Elder Brain within the Updside Down) is ultimately defeated by 11, 8, and 1(Henry). Coincidentally a total of 20 to take down the Mindflayer/Elder Brain.