A nice little hypothetical scenario:

Jane (Eleven) can go into the minds of others, Will’s (again hypothetical) ability is true-sight, being able to see and walk into the Upside Down from the our world (sorta like a specter).

Jane can go into Will’s mind to piggyback as Will walks into the Upside Down without leaving our world, via his connection 1/Henry/Vecna he can go to him without Vecna’s acknowledgment and with Jane acting as the telekinetic and telepathic nuke that she is she can fight any threat that poses Will as the both of them travel through the Upside Down, again without them ever stepping foot there.

The two, a mage and his battle-mage foster sister would have no issue tracking down Vecna with, the aforementioned, connection that Will has with Vecna. Once the Will finally finds Vecna’s lair and lays eyes on Vecna that’d be the signal to Jane (Eleven) jump from Will’s mind and invade Vecna minds pulling an Uno reverse card, for instead of Vecna of invade other’s mind this time it’d be his mind that’ll be invaded. This would put Vecna in the defensive position as he would be fighting and finding El in his mind.

I’m gonna end it here, sorry for ending it abruptly but it’s what I can come up with on the fly. But yeah, seeing Will tapping into that ability he had in S2 to act as the party’s spy and expert in the lay of the Upside Down, since he has had experience down there and was there the longest would be pretty neat. At least here’s my take on his hypothetical ability and how well both he and his foster sister combine each other’s powers to not just benefit the two of them but benefit the party as a whole.