Have it so that you can upload .png files as well, instead of .jpg being the only acceptable file format.
Scheduling, because I can't tell you the amount of times I've forgotten to update the Webtoons version at the correct time because I scheduled the episode for release on Tapas and my big monkey brain keeps thinking it updated on the Webtoons version, too.
A profile. I don't know, I just thought it'd be kind of cool? Also whenever someone has multiple comics on Webtoons they can just link to their profile instead of posting the individual links of the comics.
Have music and gif support available for Canvas Webtoons, too.
Lastly,I'd like to actually get notified WHEN SOMEONE COMMENTS ON MY COMIC!!!!