Grace Mugabe, 51, was addressing a rally of the governing Zanu-PF party.
Mr Mugabe has governed Zimbabwe since the end of white-majority rule in 1980 following a bitterly fought war.
His wife, who has often professed her undying loyalty to her husband, has assumed an increasingly high profile.
"One day when God decides that Mugabe dies, we will have his corpse appear as a candidate on the ballot paper," Mrs Mugabe told the rally in Buhera, south-east of the capital Harare.
