I wanna see Thresh start interacting with champions other than Lucien.
While I would definitely like to see more of Lucien's quest for vengeance, I feel like Thresh should have more interactions with other champions; as he tends to take souls from all around Runeterra. His bio says he goes after those with the strongest wills, and I imagine a lot of champions have pretty strong wills.
With that said, I thought it would be cool if Thresh started tearing apart the Crownguard family, likely starting with Lux and Garen's parents. Additionally, I imagine Vayne might try (and fail) to kill him if she ever comes across him. There's probably plenty more champions that Thresh would love to break and would make for some fun rivalries.