You have 6 dungeons (at least) from Vanilla that have been updated in the past in some way with new bosses, mechanics, etc.
The 6 dungeons we could use that have been updated in the past are:
Deadmines
Razorfen Kraul
Razorfen Downs
Scarlet Halls
Scarlet Cathedral
Blackfathom Deeps
For the timewalking raid you could reuse Molten Core from the 10th anniversary.
You can also add rep items for badges for factions like Timbermaw, Brood of Nozdormu, Cenarion Circle, etc. And maybe some recolors of some vanilla appearances. Maybe make the vanilla timewalking mounts be the armorless fast mounts.
I dunno, it just feels a bit strange that out of all of the Timewalking events, that we can’t go back to vanilla dungeons.
Deadmines was updated in Cata, RFK/RFD and BFD was updated in WoD with added mechanics and a new route, SH and SC was updated in MoP. I don’t think they show that much age compared to the other vanilla dungeons that haven’t seen any updates. If we can do TBC dungeons we can do dungeons that have been updated since then.
Well the good thing about Timewalking is that you build the raid yourself so you can avoid situations like with the LFR system.
Have you tried putting in a ticket? I know in the past that if they could verify that you owned the item in the past that they can reinstate the transmog for you. My friend did this for his T3 shoulders.