1) Workspaces of workspaces
Let's say I'm working on five projects at the same time. Each project usually has totally different workspace configurations.
For example:
Project1 might have workspaces 1: dev, 2: mail, 3: tail -f access.log
Project2 might have workspaces 1: photoshop, 2: mypaint, 3: shell
Project3 might have workspaces 1: trello, 2: research
...
This is like tmux sessions. I can have multiple sessions, each session has multiple windows and panes. In tmux this works very well but in i3 you can'd do that.
I'm currently emulating this by running five independent i3 copies on five displays :1, :2, :3, :4, :5 and I switch between displays to switch to another project.
2) Static tiling
I3 is a dynamic tiling manager, which means it uses the entire screen. However, once you establish your workflow, you often have windows that take 1/3 of the screen and are 1/3 off the top, etc. and you don't really want them to take the entire screen because it makes you less productive and it feels weird. Also, once you establish your workflow, you don't want the workspace to change or resize. You just know where everything is and don't want a slightest disturbance in your workflow.
This is often hard to do in i3 and you need to use tricks such as vertical and horizontal splits of empty terminals to put the app where you need it to be. And it's extremely easy to mess this up, if you accidentally open a new terminal. Then your app resizes and shifts, and when you close the accidental terminal, it's no longer where it was because of many vertical and horizontal splits.
I found Notion window manager that is static tiling but I haven't tried it. It can also have tabs inside static tiles, which makes it very interesting.
3) Run or raise
Run or raise starts a program if it's not running, focuses it if it's running, and hides it when no longer needed.
For example, pressing alt+n starts and shows a notepad if it's not yet running. If it's running, it focuses the notepad and pressing alt+n again hides it.
I3 doesn't have this feature but it can be emulated via xdotool and scratchpads.
4) Using fzf instead of dmenu
Dmenu is nice but you need to type too much. The solution is to use fzf instead of dmenu.
I'm currently displaying a floating sticky terminal window that runs fzf, gets my input, performs the action, and disappears. It's so much faster than dmenu.
5) Using fzf to switch apps, workspaces, workspaces-of-workspaces
Often, when you're working on project2, you remember you had similar idea/code/notes in project4. You then have to switch to display :4, then switch to correct workspace and find the info.
This could be simplified by using fzf and just instantly switching where you need.
6) Using alt-tab to switch back to previous place you were.
Let's say you switched from appx in project2 on display :2 to appy in project4 on display :4, and now you have to cycle between them. You can't do that right now so the workflow isn't perfect and there's a lot of drag.