One problem with trying to answer the question “What would I create if I really had Holodeck?” is that the question is woefully broad. The Holodeck is not a genre. It may not even be a medium.
Which means that there isn’t one thing or another that it particularly suggests — it suggests everything. It’s sort of like asking the question “What should a book be about?”
In fact, the analogy with a book is fairly good. The possibilities attainable in written language are vastly greater than the possibilities attainable in cinema. If you doubt that, just go back and reread Gogol’s The Nose.
Similarly, the very power of the Holodeck can make it difficult to narrow down creative choices. But I can think of various interesting directions.
Storytelling, games, construction toys, music (anything from enjoying a musical performance to creating original music, to creating original musical instruments), visits to exotic places, both possible and impossible, these are just a few of the possibilities.
Then there are the serious uses: Architecture, medicine, art, science, literary analysis, financial models, the list goes on and on.
In each case, the Holodeck manifests itself differently. To compare one Holodeck experience to another might be as futile as comparing a romantic farce to a Shakespearean tragedy.
I suspect that the Holodeck, if we ever manage to make it a reality, will not turn out to be a medium at all. Like the computer, it will more likely be akin to the computer — a “meta-medium”. That is, a vastly protean substrate through which new media are continually discovered and developed.