I have thought about...
He creido sobre que el sugierio. I have thought about what he suggested. Thank you
2 Answers
He pensado en lo que (me) ha sugerido.
Think about ---> Need to use pensar. Creer means to believe or to have an opinion.
(Me) ha sugerido ----> Suggested to me. This is even though your sentence does not include this indirect object.
I think it would be "He estado pensando acerca de lo que él sugirio"