Good idea to start. I can draw some arena for you. just give me general idea via PM.
My suggestion for this kind of event is as following.
1. You should add in some story to the fight give a reason to fight for build some town to support the story.
2. You should assemble some committee who run the tournament ,edit fighting state and draw intro story.
3. You should have the clear rule on how you judge the artist (ex. give the point base on vote or by judge).
Here is this kind of event that they do on FB in my country.
https://www.facebook.com/groups/949941655026085/ They judge the winner of each round by voting. The
committee is the one who edit the topic and operate all event. I will also sent some character to joint with you guy too. ^
^
Relate to the upper comment. In my country the tournament only limit to 1 character or duo character
per person. The one who want to joint the tournament must have their character introduction in detail as
following.
1. Intro story not least than 4 pages of complete manga.
2. Complete character design (in color) with detail explanation of their skill and move with referent picture
(because the opponent need information to draw your character in their turn ) and general
data of the character (ex. age ,sex etc.)