Highlight Reel Baseball
Baseball, but with the outfield stands pushed back about 100 feet and the Designated Hitter replaced by a Rover (an extra outfielder). Out-of-the-park home runs would become a lot less common, but doubles, triples, inside-the-park-home-runs, diving catches and throw outs and base running would become a lot more common. In other words, baseball would actually become an exciting team sport, instead of just a pitcher-vs-hitter sport that is only exciting in the playoffs.
Football (soccer), but instead of a goal net there is a circular, quidditch-esque hoop high in the air (15- 20 feet up, maybe), and the goalie crease is a trampoline that only the goalkeeper is allowed to use.
Ice hockey on a massive lake or in a football stadium. If played on a lake, there is no limit to how big the playing surface can be, or how many players per team there can be. The goalie’s net is soccer-sized — or at least, bigger than an NHL net. The ice quality is maintained by robo-zambonis
Basketball, played on a hockey-sized court (2.1 times longer and 1.7 wider than an NBA court), with hockey-style line changes (‘on the fly’, rather than during stoppages of play), three 20-minute periods per game, first-basket-wins overtime, only 1 timeout per team per game, boards instead of out-of-bounds, and, instead of free throws, a 2 minute penalty kill following every 3 fouls by a single player. All baskets are worth 1 point.
Played on roller blades, with a tennis ball, on a football-field-sized clay court. Each team has 3 racketeers (they carry tennis rackets), 2 sticklers (they carry hockey sticks), and 1 Lone Gunslinger (he/she carries a hockey stick, but also has a tennis racket slung over his/her shoulder, to unholster and re-holster at will). The goal can be either a soccer net with a goalkeeper, or a raised hoop with no keeper.