Discovery has such a small crew, any new number one should logically come from the bridge crew. My biggest issue is keeping Saru as captain. Originally, Discovery was a science vessel in a time when the universe was much more settled. In that setting, Saru is a decent captain, although I think he functioned much better as a number one himself. Now, however, it appears that Discovery needs to be more of an all-purpose ship with the potential for getting engaged in armed conflict. I don't think Saru has the "it" factor, the factor that made Kirk, Picard, Janeway, etc., great captains. YMMV.