Add timestamp to a post

Set great pagination to posts and comments

Add unit tests

Add ability to delete email account

Send notification to user whose post was commented.

Handle storage when network is unavailable

Add photo and username to comments

On full screen add the ability to share or download images.

There is a crash while pressing on comments icon

Replace AsyncTask with Coroutines

https://material.io/tools/color

Replace fields on sign up screen with radiobuttons, date dialog, etc.

Add feedback while uploading posts or comments

Replace toasts with snackbars

App with tabs crashes when device changes its orientation.

Create new instrumented tests for all screens. Create walkthrough. Use Kakao.

Do something with profile and settings screens :)

Add recommended events to Firebase Analytics

Change view card while horizontal orientation. Variant: off screen rotation :)

Add section "about"

https://my-json-server.typicode.com/

Find out how to get user's gender and birthday

Add authors to posts

Separate posts and comments queries

I saw that you're using the Retrofit, although you are also using custom serialization. Why? Maybe add the GsonConverter when you build the Retrofit instance. This will make your code much more readable.
Maybe add some dependecy injection as well. Since you are using Kotlin... maybe add the Koin library. The setup is far more easier than Dagger 2.
I can help you with it. Let me know if you are interested.

While adding new comment/post pagelist updated from the 1st page

Current RatingBar works wrong on the device. Needs to create custom RatingBar with N ImageButtons.
Example https://stackoverflow.com/questions/14120419/how-to-create-a-custom-views-layout-programmatically

Check password restore

Refactor post adapter

Extract auth functions to another class

Add google and vk authentication

Remove google-services.json from the project. Find out how to run circle-ci without it.

https://material.io/tools/color/#!/?view.left=0&view.right=0&primary.color=6002ee&secondary.color=FF9800&primary.text.color=ffffff

Make logs more useful

Add guest login

Add sentry to application

There is no photo in circleimageview

Create network services and Gson converters with factories

Replace login, splash and signup activities with fragments. Add animation on fragments transition

There is a crash while clicking on inloaded post

Need to upload 10 posts per request. Set comment caching.

Rebuild main screen. Build tabs:

1. Profile
2. Feed
3. Settings

App revision for bug search

Create new entities for work with image store

RecyclerView scrolls slowly maybe because of large images. Find it out and solve.

Replace Realm with Room

Add signing to apk