How about Admiral Picard?
Sisko was way out of his league commanding a fortress of galactic importance. Promoting him further beyond Captain so soon would be somewhat unrealistic. But Picard was overdue a promotion anyway; taking him out of the TNG equation would free that show to explore all-new dramatic venues, while inserting Sisko's old nemesis into the DS9 format would make for some interesting clashes.
Picard would probably be little more than a guest star in this proposal, filling the slot of Admiral Ross (with a little less brass on his collar, but quite a bit of dramatic impact to compensate for it). We could still have Sisko doing the "Realpolitik" where Maquis scum needs to be bombarded and alliances switched at the drop of a hat, while his desk jockey superior would actually be the one holding up the flag of UFP moral superiority - a welcome reversal of roles.
Timo Saloniemi