动词适当形式填空I wish I (be) a host of a charity show.

I wish I were a host of a charity show.
wish后面的宾语从句要用虚拟语气。虚拟语气中，was常改成were。

I wish I (were /was) a host of a charity show.
wish 后面的宾语从句用虚拟语气。表示不能实现的或者不现实的愿望，要求

I wish I were a host of a charity show.
wish后面的宾语从句要用虚拟语气,从句中系动词be的形式是were。

个人的答案：
I wish I could be a host of a charity show.