Vida Blue was the first one I thought of. Bowie Kuhn probably cost the Reds the 1978 NL West title when he vetoed that trade. If the Reds win the West, Sparky probably remains the manager.
George Anderson mentioned the possibility the Reds could have obtained George Brett for Tony Perez prior to the '75 season. Obviously Brett was a greater player than the Big Dog and much younger. Having Brett might have brought the Reds some titles in the late 70s or early 80s. However if the Reds make that trade do they then leave Rose in the outfield and fail to start Foster? Does Brett hit a Bill Lee blooper pitch in game seven of the 1975 World Series over the Green Monster? I am loath to mess with the 1975 BRM.
One mythical trade I have thought about would have been prior to the 1972 season. Gussie Busch decided to trade Steve Carlton from the Cardinals because the beer baron was angry that Carlton wanted a raise. Carlton was ultimately traded to the Phillies for Rick Wise. Wise was a 26 year old righthander with a career record of 75-76 and a 3.60 ERA; he had completed maybe his best season in 1971, going 17-14 with a 2.88 ERA. I have wondered if Bob Howsam could have tempted Busch with Gary Nolan, a not quite 24 year old righthander with a then career mark of 61-42 and a 2.98 ERA. Nolan had gone 12-15 with a 3.16 ERA in 1971 and was a year removed from a 18-7 campaign. Throw in Bernie Carbo if necessary but get Carlton to anchor the staff.