I see a lot of people having a pretty clear idea of a specific thing they want to do when the theme hasn't even been revealed. Is this the standard? Is everybody doing this? Should I?

its pretty normal for people to have ideas or characters or plans to work with someone etc when preparinng to join a jam. but with the theme not announced yet, obviously planning too much is detrimental since ultimately the ideas will have to match the theme and then theres a risk you gotta make big changes. so like dont worry about it. if you have some ideas let them float in your brain but the real work starts when the jam begins. the duration of the jam is long enough theres no issue even if you only start thinking once the theme is announced