Hi Cristian, welcome back 
There wouldn’t be much space for titles and categories’ descriptions which are enabled by default for anonymous and trust level 0 users. A mockup using a “real world” example would be great to see how it would work.
That’s not the same but I’ve modified the mobile category page layout on my own forum from:
to:
But I think it could be improved even more with some grid view as you suggest and/or removing the three topics for each category.