Here are a few features that I think would be good to add:

Administration & Moderation

• Signup applications (under the Reports button in Inbox would be ideal)
• View votes (afaik mods should also be able to view votes in their communities)
• Perhaps some sort of administration panel
• Merge "Admin tools" and "Mod tools"
• View moderation & report history for users
• Better modlog UI
• Checks for whether or not some mod actions can be done on a user and hide options that can't be applied
• Add "Feature Post in instance" to posts

Create new post dialog

• Rename "Compose post" to "Body" or "Text"
• Pre-expand text field area
• Add new field for thumbnail URL
• Allow choosing post language

Create new comment dialog

• Allow choosing comment language
• See who you are replying to in addition to the comment content

Media

• View & manage uploaded media
• Add option to loop videos
• Add option to unblur NSFW images when in NSFW communities

Other

• More user friendly errors
• Advanced search (search in community, filters, etc.)

Oh and the updated join-lemmy.org applications page lists that Summit has 2 GitHub repos. I think it would be less confusing to just link to the repo that contains the source code instead.