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Revert Baton Pass so that it passes Mean Look. That by itself was pretty much the only reason it was OU in Gen 4. Beyond that, adding bulk in the form of a deeper movepool and better abilities will help. The biggest problem is that the metagame right now is so offensively-focused that stall teams need to be perfectly made in order to be useful, and Umbreon just does not have the tools needed to stall properly. Cloyster has high defense and low attack/special attack, but having Shell Smash turns it into a viable OU sweeper. Maybe give Umbreon a copycat dark-type move?