Do you have any plans of adding these in nere future?  

The "3" already exists.

"2" is enabled when you implement tags.

"4" No thanks. Almost everybody on a mobile device has terrible spilling and reading concussion. In short order you will have tag cloud filled with bullsh1t. Been there done that.

"1" Topic/post bookmarks is on my wishlist too.

1. Mark topic as favorite so that user may easily refer later - (in to-do)
2. Add Related Topics - (exists, just use tags to see related topics in topic footer section)
3. Pre ticked checkbox for "subscribe to topic" - (exists, the option is in Settings > Features Tab)
4. Search Topics while typing topic to avoid duplicate topic - (in to-do)
5. Force users to add tags (not planned)
6. Inbuilt notification system so that users will be notified if their topic is replied, liked, voted, answered or commented - (in to-do)
7. Adding users as a friend or following them - (thank you. we'll take this under consideration)