A compilation of all gear locker objects found in the resource size table which provide insight on gear that could be returning down the line. A more detailed explanation is provided under the lists.
Every released gear in Splatoon 2 (+the Octoleet Armor and Octoleet Boots) seems to have been automatically ported to Splatoon 3, left in the RSTB (Resource Size Table). Since gear that would likely never return is even in there (such as the SWC Logo Tee), it was considered unreliable to use it to check what may actually return. However, there are ones that have a model and actor in the RSTB, that are missing a locker actor in the RSTB, and what's missing is consistent too. For example, every gear that was part of a promotion (such as the Record Shop Look EP, Eminence set, or Online set), or was received from Octo Canyon/Octo Expansion (such as the Hero Hoodie Replica, Octo Layered LS, or Old-Timey Clothes), lacks a locker actor. Because of this, it seems to be decently more reliable for checking what may return. There are 3 exceptions to this however, the Fresh Octo Tee (Octo Expansion Reward), Sesame Salt 270s, and Sea Slug Volt 95s (Both based on NIKE shoes).
This isn't a 100% proven list however, and is fully subject to change.