A good portion of that is actually already in the game. Reordered your list a bit to better structure my answer.
- better replay indexing (tag replays automatically for decaps, or custom manual tags) and custom parameters for auto-saves (again, decaps for example)
- maybe the ability to comment directly under replays, tbd by the uploader - moderated ofc
These have been a thing since 5.33 (released in Sep 2018) - you can move your replays between folders, assign tags and share them online by uploading to server. Uploaded replays can be rated / commented, too.
Custom conditions for autosaving sound nice but I cannot say anything specific whether it's going to be added in any near future.
- better automatic mod downloads so there's less replay glitching, or make it clear from access point that you need a certain mod, and make it visually easy to click a button, many new players don't know / commands
When downloading replays with game client, it also downloads the replay mod if it's missing. Won't be the issue for tbn as it has mods included in replay files.
- an integrated "clip" function, to make gif-like thumbnails in the browsing menus, click it to dl/view the replay as normal
This one isn't going to happen, a gif essentially takes as much space as the whole rpl file. Having a single screenshot to showcase the replay sounds a bit better but you can barely tell what the replay is about by a single screenshot - a proper replay name and tags would be just as descriptive.
- personal replay pages integrated with replay/player browsing menus, in which players can customize what replays appear and in what order. Almost a splash page for players, with a small static image of their textured tori
- an actual, functioning friend mechanic, that can take you to their replay/splash page
Solid ideaa, may be added in future updates.
- Heavily improved replay browsing, to see top replays this week/month/all-time
Sounds good, doesn't work
But yeah, this actually does sound good in theory but won't just start working as expected if we add a way to view the top - and that's the core issue here.
Players need to have an actual reason to go watch recently uploaded replays to rate them and thus form some sort of rating. Current system obviously doesn't really work as replays barely get any downloads (and even less ratings) in that sense. The solution may be to introduce "watch N replays" quests but imo that would be just annoying for players - this is the sort of thing mobile games would do to give you some e-bucks.
If you have any suggestions regarding that, please share your thoughts.