I would probably keep Malwarebytes free for on-demand scans. As others have mentioned you can also add Norton Power Eraser and Emsisoft Emergency Kit for on-demand scans as well. Depending on your experience you can also look into adding Voodooshield and OS Armor as companions to WD. They function differently than Malwarebytes, but they offer great protection as a companion to any AV. Best of all you can get both for free. Another great suggestion is to use Configure Defender to further strengthen WD. Another option to consider would be to switch out WD for either Avast, Avira, Kaspersky free, or Bitdefender free if you don't feel comfortable running WD by its self. Another option that just came to me as I was typing this would be to add Comodo Firewall free with CS settings, that will really sure up your real-time protection.
There are some great free real-time options, I would suggest giving some of these a try!