Seeking:
When seeking, show [current-time]/[overall-time] along the seek OSD bar.
The reason for this is basically the same as the current volume OSD (numeral value + bar). In many occasions it helps to know exactly what value you're at, this is especially the case in non-desk setups, such as a PC connected to a TV and so on.
OSD makes it clear enough, with the other option we have right now is adding time to the float control bar, which can be unreadable from a distance.
Show file name at start:
Basically to show the filename whenever you open a file a couple of seconds. Similar to the current audio track OSD at the start.
Reason for this is similar to seeking, however it is also useful in playlists playing in fullscreen. If for example you decide to binge watch a show on your TV, after a while one can lose track.
While the current show info (Shit + I) is definitely helpful, it fills up the screen and requires more interaction, similar to that of "Stats for Nerds" on Youtube.
A simple 1 or 2 seconds flash of "Media title, if empty show filename" would be great at the beginning of each file.
Associated files icons:
This is something that isn't really needed for Linux, since adding an icon pack to the operating system or to specific MIME types is rather common.
You've mentioned that you would only want to use open source icons for SMPlayer. If so, I would highly recommend using the Papirus icons set, of which you already include their SMPlayer skin. They're open source and well maintained, though for MIME Types you can use a singular icon as you do now or multiple for each type.
Important note about this post:
I fully realize that SMPlayer exists as an open source project and is developed in its entirety by dedicating free time to provide something for everyone to enjoy.
I sincerely thank you for your efforts.
In no way I am demanding these features to be implemented, but I definitely wish for them. The reason I created this post is because I found myself viewing and sometimes even replying to older posts, which felt wrong (reviving dead topics).
I definitely hope I could help push patches myself, though it will take me quite sometime to even do basic things. I'll absolutely try though, so wish me luck.
Keep up the great work.