Can Clouds eyes become slits after the events of ff7 Advent Children?

i guess technically, they could if Sephiroth came back and somehow managed to take control of Cloud again as he did in the original 7. but really no because i mean, if you played the original 7, then you would know that Sephiroth controled Cloud through the mass qunatities of JENOVA cells in his body, making Cloud give Sephiroth the Black Materia. however in the end of the game, before the scripted fight between Cloud and Sephiroth, Sephiroth attampts to take control of Cloud's mind again, but due to Cloud's new found strength, he is not put under Sepiroth's control. so, in answer to your question, NO.

Signed,
Phoenix