This was a question on my mind. If the Non-slave states of the North (We'll just call them the Free States of America) seceded from the rest of the US, who would the people choose to be the first President of the Free States of America?
Depends which ones do so.
Ohio, Indiana and Illinois still regard the Mississippi as important economically, so are unlikely to secede[1]. Seward is a possibility if NY secedes, failing which it is probably someone from New England. Sumner maybe?
[1] Didn't someone (Henry Clay?) declare that "Massachusetts and South Carolina, it may be, can agree together on an acceptable border between them. There is no way that any such line can be drawn through the West"?