In Super Mario Galaxy 2, Mario collects yellow stars and then he collects the big yellow stars known as the Grand Stars. Later on in the game, Mario starts to collect green stars. But there is no Grand Green Star in that game. I think the makers of Super Mario Galaxy 2 made this intentional to imply that the Grand Green Star will be featured in a future Mario game. I think it would have to be the ultimate, special, secret, hidden, legendary star.
I would imagine it having a special appearance such as magical sparkles and energy all over it. I mean, the way I see it would be like this. There is just 3 green stars in Super Mario Galaxy. But the green stars were proven to be very significant as there were many of them in Super Mario Galaxy 2. Therefore, it would follow that there would be a future Mario game proving the green stars to be even more significant as they feature the Legendary Grand Green Star.
I would imagine it having a special appearance such as magical sparkles and energy all over it. I mean, the way I see it would be like this. There is just 3 green stars in Super Mario Galaxy. But the green stars were proven to be very significant as there were many of them in Super Mario Galaxy 2. Therefore, it would follow that there would be a future Mario game proving the green stars to be even more significant as they feature the Legendary Grand Green Star.