Discourse if very fixable software but i need to know how far we can go with it, and check if its possible to do the below:
1- Trending widget on side bar: best topics or categories’ in the last 24 hours or more.
2- Users can subscribe for specific categories.
3- Costume feed homepage, so the users only see the posts from the categories they subscribed for, not all all the posts.
I believe there was something like that mentioned in this topic.
@Justin_Vega proposed something slightly similar to Pavilion in a topic on their forum.
Not enough data to how to do it 
also want it. Something like discuz have.
Yes, you can build all three, but they require different levels of customization in Discourse.
For the trending widget, Discourse doesn’t give you a ready-made “trending in the last 24 hours” widget with custom ranking logic. You’d need a theme component or custom development to decide what counts as trending, such as recent replies, views, likes, or activity.
Category subscriptions are much easier. Discourse already lets users follow categories, so you can use that to determine what content they are interested in.
For the custom homepage, you can then build a feed that shows topics only from the categories a user follows instead of showing the entire community feed.
So the setup would basically be:
Discourse category following → user preferences
Custom feed → topics from those categories
Custom trending widget → recently active/popular topics
If the main requirement is a more visual or customizable content feed, you could also use something like Taggbox for the presentation layer while keeping Discourse for the actual community and discussions.
I wouldn’t use a widget to replace Discourse’s core functionality. I’d use it where you need more flexibility in how the content is displayed.