Was just thinking about how generalized tunnel access--while not a thing in the near-term because I think it could be a big map balance bugbear that could take a while to really figure out--will probably eventually come, but I don't know about it being an item-based thing like Shrink Ray. It doesn't seem powerful enough to warrant dedicating a full active item slot to it. The other utility of making yourself small for fights seems very frustrating to balance too.
If there was a map powerup placed near some tunnel entrances that took a short bit to acquire via minigame--think doing a Sinners, or maybe it's just a rune--that would shrink you briefly and allow you to enter a tunnel. Maybe it's a piece of the Rat King's cheese. Then, no matter what size you were, you would be able to "pop" out of an exit, any exit, as long as you were already inside a tunnel. One-way exits would also solve the reappearing issue of people getting stuck in tunnels like Paradox etc. and having to Unstick.
EDIT: The other advantage of the powerup taking a few seconds to acquire also means that Rem, Calico, and M&K having the innate ability to enter the tunnels remains strong compared to other heroes. Rem can outrace a pursuer because the powerup would take too long to get.
If there was a map powerup placed near some tunnel entrances that took a short bit to acquire via minigame--think doing a Sinners, or maybe it's just a rune--that would shrink you briefly and allow you to enter a tunnel. Maybe it's a piece of the Rat King's cheese. Then, no matter what size you were, you would be able to "pop" out of an exit, any exit, as long as you were already inside a tunnel. One-way exits would also solve the reappearing issue of people getting stuck in tunnels like Paradox etc. and having to Unstick.
EDIT: The other advantage of the powerup taking a few seconds to acquire also means that Rem, Calico, and M&K having the innate ability to enter the tunnels remains strong compared to other heroes. Rem can outrace a pursuer because the powerup would take too long to get.