There are common closings such as "respectfully", "Best Regards", "Yours truly" etc. but if you have reason to make your own is that allowed or is it more of a formality that is set in stone to used a prescribed one? For example if one is writing an e-mail or letter to people they will be playing soccer with, can they end with "See you on the soccer field"? Also how should the grammar be treated? The first three examples I gave are followed with a comma but if I can use my own made up sentence would it follow the normal rules of grammar (e.g. See you on the soccer field.)?
[Answer describing UK culture]
Nothing is set in stone. What close you use depends on the formality of the communication and your familiarity with your correspondent.
With a good friend, I might sign an email (or even a written note) with just
but if I were writing a formal letter to a bishop, then there is a set convention:
Even that can be varied; and there are many degrees in between.
"See you on the football field" (with or without an exclamation mark) is informal enough for a group of friends who don't need even a low level of formality like "Regards".
The usual complimentary closing to a formal letter usually ends with a comma because your name (in the form of your signature) is considered to be part of the same sentence.
While you are free to use any form of words that suits your relationship with the recipient and the content of the letter, there are some sentences that may look odd when ended with a comma.
See you on the soccer field almost begs for a full stop because it is a complete thought in itself. One way to retain the convention of ending with a comma (so that your reader does not think that you do not know the convention) is to amend the beginning of the sentence.