The umpire statistics were quietly and informally retired after the 2010 season, but you asked us to bring them back and that's what we've done.
In addition to statistics for the current year, you can now access historical seasons as far back as 1950.
Umpire statistics can be accessed by clicking Statistics > Sortable Statistics in the top navigation menu and selecting "Umpires Report" under the General heading. Or, you can click right here to access them directly.
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The big one would be merging umpire information with PitchFx data to see the percentage of strikes and called strikes. This is perceived to be the largest impact that an individual umpire's style impacts a game.
I noticed you can add Balks via 'Statistic Selection', but it should make the standard report, as it's a more direct decision of the umpires. These are (too) rare, but multi-year trends may emerge down the road.
Ejections (managers or players), if the data is available.
Base stealing information (SB, CS, Pickoffs) might be a good illustration of what pitchers feel like they can get away with.