I want to use "I hope it will be great" as this positive, supportive, and showing your excitement for the other sort of statement. Not exactly sure how to describe so I will give a few examples. Like when your friend is going on a trip and you say "I hope the trip will be great". Or they are going to this concert, burning man, and you go "I hope you will have fun".
The problem for me with this "I hope it will be great/you will have fun" statement is that it does not seem appropriate for occasions when it is 100% clear to everyone that the said thing is going to be terrific. To clarify, the statement "I hope it will be great" is appropriate for the case when someone is taking an exam. An exam may go well or may not, it is not at all normal to expect that the only outcome for the exam is a 10. Therefore it is appropriate to wish that the exam would go well. But for the situations, like with the trip or the concert, where the context is that it is clear to everyone that both of these events will only be completely great, wishing further on them to be great seems redundant and more importantly implies that I think that for some reason these events may not go well, hence the wish.
It is a nuanced thing I want to express but I feel a lot of people likely run into a similar problem. Where they want to be supportive and show their excitement for someone with regards to the occasion which is 100% considered to be great/fun/etc., without appearing like they are not completely behind the said notion.
Best Answer
If you want to wish someone - who is off to have a wonderful time somewhere - a wonderful time, you say:
Bon Voyage!
(An "oldie but goodie" borrowed from the French but, common enough to be considered part of the English lexicon.)