Hi,
I own 2 large football news websites (each 200.000 visitors/day) in small countries.
Who has some tips to maximise revenue? Both websites have 90% returning visitors...
What are you doing with those sites in terms of monetization now?
Hard to suggest any improvements when we dont know what you have there now 
Try and hook up some direct deals with sports betting sites. They pay top dollar if the quality is good
Hello,
To better monetize your websites, I would say that you can promote offers directly related to football world and news and additional offers that could interest your traffic as well such as Games offers , certain Health Offers (muscles for ex), Dating or Sweepstakes and more.
PM me if you wish to get further info about Hot offers which could be integrated into you website.
One more tip for you regarding football stuff, from what I know promoting from the articles themselves is a powerful marketing action.
As per the website itself, you can consider to combine different promotional methods to increase the traffic volume and interest. For instance, you can facilitate and encourage your users with marketing strategies (especially since most of they are returning) to share about it to their friends on social platforms.
Cheers,
ilana