Do you think vote sould be private ? Public ? And why ?
Public. Lots of downvotes is information that could indicate that the commenter is lying, or just saying something unpopular. But either way, it's information. Before youtube started hiding downvotes, it was easy to tell that a video had a misleading title based on downvotes. Now clickbait dominates the platform.
Are you sastified with the current voting system ? And why ?
No. I agree that the slashdot method with more than just upvote/downvote is better. In a perfect world I imagine we could have every emoji be a reaction option, and then you could sort by putting an emoji in a bar at the top. In reality I imagine this would be a challenge from a backend perspective, but maybe like the top 5 or 10 emoji reactions could be an option for selection.
What other interesting software/website that tried something different do you know ?
I'll do the opposite and say - please do not remove downvotes like Twitter/Bluesky/mastodon etc. Downvotes are super important. People need to be able to boo, the only place people aren't allowed to boo are in church or at cult rallies. And that's why those platforms are especially bad for misinformation, hyperbole, and overall depravity.
What way do you imagine to highlight content and improve search, discoverability ?
Remove all as a forced/default option on the main page. Back in the day before reddit had r/all, communities were much more diverse and niche, and this helped separate communities flourish in their own way. When r/all was added, the content started to resemble twitter, if not just becoming screenshots of twitter, on just about every sub. This actually improves discoverability because it would force users to branch out and look at subs instead of just looking at what's on all.