Clearly it has to be someone who was not intimately involved with the "ancien regime" that has been revealed as corrupt from top to bottom. This disqualifies Coe - who either knew and did/said nothing and is therefore as guilty as anyone else - or broke bread with the corrupt ones and noticed nothing was amiss in which case he displayed an amazing lack of insight, judgement, perception - which should also rule him out!
The sport will have to go to an outsider, possibly someone completely unattached to the sport in any way. Pay them a decent salary, monitor their expenses very closely, make it a rule that nobody in any position of importance serves for more than 5 (?) years. Do not allow any family members to be appointed to influential positions, do not accept any sort of commercial links with big sports companies. Have a body of external auditors scrutinise the biding process for any championships.
Many of these points appear pretty basic, even petty, but the has to get back to this level of simple, open government.