Explore what you have.
Don't perform some micro trick which you could perform up close *thread*.
Exploit this oppurtunity. You have a giant field to your disposal to perform one amazing trick/ routine.
I would suggest not performing a trick you already know or something thats on the market per se as nothing will really satisfy it. What I suggest is to look into every aspect of this football field and see how you can use it differently. Then use old concepts and adapt it to a football field.
Dont perform something which you could perform else where. You are given a great oppurtunity to use somehting you normally dont have. Why revert back to cards and strings?
You said you wanted something "new". But that is really hard to find nowadays. Why not take something old and make it new.
I would generally suggest anything large scale. Mentalism hits hard, so why not perform some form of a large scale prediction involving a large amount of the audience. A teleporting man from the centre of the field to somewhere obvious would be fun. Back to that new comment. Why not perform a teleportation/ metamorphosis.
Just off the top of my head. You are put into a sack, in a box. The audience are told that you are trying to escape. You dont. Someone opens the box and out pours.... a truck load of footballs, or perhaps helium balloons. You are then found to be somewhere odd, as in lets say the commentators seat talking into the mic, or you could be the person who opened the box. Nothing new with this idea but it can be used to fit your venue and situation. Prettty sure majority of your audience have at one point or another seen Copperfield perform something of the sort, but they will not make the connection as it is different in context and situation.