I am pretty confident that all of these different categories are here to stay primarily because they have become popular and have attracted artists from various genres of art and I am expecting them to add more categories as time passes.
With about two or three years of running many art contests, Roobet is like in following lists of many artists who only wait for announcement from Roobet to create their arts and join contests.
Rewards might also increase in order to makes these art contests more competitive.
It won't be a must and I see in Roobet's art contest history, prize pools are different. I guess it depends on their assessment about important of each event which will convince them to allocate bigger or smaller prize pool.
Higher prize pool is more attractive especially in times of inflation but it is not a must requirement. Even with low prize pool from past contests, I see it is attractive enough. Additionally, they don't have much competitive contests in Bitcointalk.