So I had a idea recently. What if the party was a sport team and the campaign was their path to the championship. What rules would you put in place for combat and howbto traverse this new added combat style?

I'd really have to consider which sport was being handled. I'm also not sure how magic would factor in.

In games like Soccer and Handball, I'd really make it about attack rolls. If a player is passed the ball, they have to move before they can pass or shoot (following short and long range rules). They cannot shoot unless they have already moved. The forced motion better allows for attacks of opportunity which would steal the ball and make that player the active player (and immune from opportunity attacks from the player they just stole from).

For each player who won't have the ball, they move and ready an action.

Passing and shooting uses an attack roll. This roll should be strength. Goal Keepers and players being passed / shot through should be allowed to contest the pass / shot with a dex check. On a tie or a success, the dex player catches the ball and becomes the active player.

Relevant skills can be used as a bonus action. You can use each skill in which you are proficient a number of times equal to your proficiency bonus.

Athletics increases your walking speed by 5 × Str Mod.

Acrobatics gives an opponent disadvantage when it comes to stealing the ball.

Sleight of Hand can be used to cheat / trip your opponent.

Insight can be used to guess what the opposing team is going to do that round.

Perception gives a bonus to a Goal Keeper, or character being passed / shot through.

Deception is used to gain a bonus to a pass or a shot.

ok your right like me clear up what i mean. a basic get the object to the goal. fight for object and score points per. combat like normal dnd is still allowed magic an all. killing is allowed but has penitlys as a team loses a player

A sports-themed campaign a la season 1 of Legend of Korra or something could be an awful lot of fun. As a DM, the first question I'd ask is how much focus I want to put on the actual in-game action.

If you want, like, 90 percent of the campaign to be what happens on the field/pitch/court/whatever, then yes -- make it combat-based, come up with rules for how to use skill checks to spot open teammates, steal the ball from the opposition etc. Use a full tactical map with movement rules, etc., and be prepared for one game or match to take up most of a session. (Now I'm starting to have Blood Bowl flashbacks.)

On the other hand, if you want to leave more session time for what happens off the field (practice, finding sponsors, placing bets, shady characters bribing them to throw a game, league politics, beer and wings after the game at their favorite tavern, overly enthusiastic fans, etc. etc.), then you might want to scale back how complex it is to play out a game. You could make it something as simple as a series of skill/ability checks to score a goal -- perhaps the total DC needed to score is 50, and you have up to three or four checks from different teammates to reach it or you lose possession of the ball. Something that keeps everyone involved, but is resolved a lot more quickly than a combat equivalent.

Or you could do a mix -- quicker resolutions for early-round matches, but playing out the full game for the championship. Or quicker resolutions to start a game, and going to the tactical map if it goes to overtime.