Not really able to find stuff like this, but the recent additions to Legion Timewalking for the new Mage Tower challenge that gives a Tomb of Sargeras Mythic recolor for each set relly did make me wonder what else would make me excited.
I would for instance absolutely kill for a Brown w/ dull iron recolor of the Nighthold hunter set, or a really dark or even black version of the Mythic set.
Really I hope Blizzard adds on to timewalking going forward to add unused weapons or possibly even make new recolors of old sets to give as prizes.

I have never done the Black Temple timewalking raid, but I would be tempted if it gave a unique transmog for instance. Island expeditions did give the blue recolor of the Firelands Paladin set, but I would love the rest of the armor sets from that raid with a similar recolor.
Even outside Timewalking raids or even timewalking in general. I would absolutely adore if Blizzard used the wealth of unused assets and asset flips they have available and give them as rewards for doing obscure challenges.
I am not much into levelling for instance, but you can bet your ass I would level a Trashgnome through the Shadow Priest experience on a boring levelling track like BC if it gave me a unique set. There is no limit to the dumb stuff I can imagine doing in PvE if it gave me something like the recolored hunter sets I mentioned above for instance.