Hi FranFran
as you asked about Marrakesh specifically I assume you have discounted other areas. I think Morocco as a whole is a good investment as long as you choose the development carefully. But, I still believe Marrakesh is the safest bet. It is proven, all the new coastal resorts at present have NO tourists. You can do all the predictions you like, but this can only be guesses based on more guesses.

It probably wont go up as much as the new resorts (IF they are sucessful), but I do think steady annual growth is likely.

I have also heard Essaouira is a nice place with lots of potential, but that was for redeveloping old medina riads, not sure about new developments there.

There is another thread "Marrakesh" where people have discussed particular developments.