Being able to see the defender's submission from the open hacking page. It is kind of annoying that you have to open the person's profile and then go to his submissions just to see his/her submission.(ADDED thanks to Mike!)
Fixing illegal contest ID bug when trying to hack a person through submission window in open hacking.
Splitting the hacking log(open hacking) into pages. The log sometimes can be huge which would make loading the page slower on low-end devices(like mobiles).(ADDED thanks to Mike!)
Standings search only searches through the current page which is not a public search. Making it a public search would be a lot better!
Improvement for codeforces search. Instead of searching for the exact word/username, it would search for the nearest result. Searching users' handles like this would be a lot easier.
Improvement to submission window on mobile as it is shifted a little bit on some types of mobiles.
Make GUI(especially buttons) a little larger to prevent mistakes(clicking a button by mistake like upvoting/downvoting)
Double tapping the screen on a person's submission in the standings causes the screen to zoom in. Instead, It should open the submission history window.(suggested by nutzboi)
- Being able to delete problems from mashup instead of just hiding them.
Fixing facebook support to codeforces.
Adding a method to add bio to your profile.
Overall, codeforces is a great platform and those improvements would really help! If you have any suggestions for improving codeforces, comment it down and I will add it to the above list. Upvote this blog for MikeMirzayanov to see and care about it!