Show multiple subtitle tracks at the same time
Feature request that would be useful for people who are trying to learn a language by watching subtitled videos. In that case it's useful to be able to see subtitles in two languages simultaneously (one language you're learning, and another, which you know). Could also be used when multiple people (who know different languages) are watching a video.
When displaying subtitles in two languages at the same time they would have to be positioned accordingly - still in the lower part of the screen, but one will be a bit higher than the other. Font scale would have to be appropriately reduced to allow twice as much text to fit on the screen (this may or may not be adjustable in the preferences). For simplicity, you set up the number of subtitle tracks you want in the preferences (2 is the default; probably 3 at most - anything beyond that is impossible to fit), and then the usual VLC menu would have Subtitle->Sub track N1 and Subtitle->Sub track N2, allowing you to select two tracks at once. Track N1 will be higher on the screen, Track N2 will be lower on the screen (thus, there's no need for extra UI or preferences that could have been used by the user to decide which track is placed where; everything works intuitively, more or less).
Subtitles from different languages (tracks) would have to be shown in different colors (that may also involve different background colors). THAT would definitely need extra UI elements in the preferences.
Sub-feature request number 1: Show subtitle history (also with multiple languages simultaneously). When you pause the video, and subtitle history is enabled (an extra checkbox in the preferences), VLC should draw a semi-transparent overlay that shows (in smaller font, obviously) certain number (preferences-adjustable) of past subtitles (with or without timecodes), in 2 (or 3) languages at the same time (again, appropriately scaled down and drawn in different colors). This is almost exactly the same as #25003 (if you add a scrollbar to be able to scroll up to see even more of the past subtitles, or scroll down to see the future subtitle entries, and add some UI to be able to search them, you essentially get the subtitle search feature from that feature request).
Sub-feature request number 2: Make current subtitles and subtitle history interactive (i.e. text should be selectable, it should be possible to copy it, and it should be possible to right-click it to call a context menu; ideally, the context menu should include things like dictionary lookups, but that is probably outside of the scope of VLC - still, something to keep in mind). This is almost exactly the same as #7082 . Currently-displayed subtitle entry (or entries) would probably have to be erased from the screen and put into the history (probably marked somehow to make sure the user understands that that this is where playback currently is), and then re-drawn on the screen again when the video is un-paused. Not erasing currently-displayed subtitles would be tricky, if we allow the user to scroll down to see future entries. Or maybe arrange the list in such a way that currently-displayed subtitles would just fit in the list in their current position (possibly, scrolling up or down would also make different subtitle entries become "current" (with appropriate visualization), and un-pausing the video after that would essentially rewind or skip forward accordingly).
Obviously this would only work well with text subtitles. With graphical subtitles it might be possible to do some things - multi-language display, if scaling is done correctly; maybe even subtitle history, although history would probably require subtitles to be linearized (text subtitles often have newlines inserted into them to improve readability, but in subtitle history it might be necessary to remove these newlines to ensure that entries occupy less height and more of them fit on the screen - that can't be done for graphical subtitles). Obviously, graphical subtitles can't be interactive (although copying them as pictures is better than nothing, i guess), and searching them is not possible.
An idea about playing multiple audio tracks at the same time sounds interesting (would probably need some processing, like making one track quieter and/or mixing them down to mono and playing them in separate left/right output channels), but completely outside of the scope of this particular issue.