写句子要完整，不要缺胳膊少腿。加上主语：

I think someone can do more than may be possible.

than may be possible  是比较状语从句。因为主句含有 more ，便引出than 从句。

从句中，暗示 than 是 may be possible 的主语。一些类似表达：

He never says more than is necessary.

他从不多说没有必要的话。

He often eats more than is good for his health.

他吃的饭量往往多于对身体有益的量。