My guess is that it will be former President, as one retains their highest title until their death. This is the reason that we still call former presidents like Jimmy Carter, HW, and Bill Clinton, "Mr. President." Moreover, First Lady (and/or First Gentleman) is not an official title of the US government.
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Thor:
Intel Core i7 4770K|ASUS Z87Pro|32GB DDR3 RAM|11GB EVGA GeForce RTX 2080Ti Black|256GB Crucial M4 SSD+2TB WD HDD|4X LG BD-RE|32" Acer Predator Z321QU 165Hz G-Sync (2540x1440)|Logitech Z-323 2.1 Sound|Win 10 Pro
Explorer (MSI GL63 8RE-629 Laptop):
Intel Core i7 8750H|16GB DDR4 RAM|6GB GeForce GTX 1060|128GB SSD+1TB HDD|15.6" Widescreen (1920x1080)|Logitech R-20 2.1 Sound|Win 10 Home
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