THE ONLY WAY, the character Eddie can come back, will be as a cameo, if, S5 has time-travel. Like the party goes back in time and saves all(farewell and stuff) and the universe resets to 1983 with Will not going missing.
Fastforward to 1986-1990, we see all characters living a happily and Eddie comes as a cameo playing DnD and Mike says "Stranger Things have happened" and get a deja vu moments or a connection from an alternative timeline.
Thought it is not recommended, Timetravel will have some effect in S5.
One/Few series has done it before. (Will not name it to avoid spoilers).