François Bayrou has been giving signals that he might run. He's also been telling François Fillon to drop out because he's too tainted by "the money power" to stay in the race. Presumably he feels the same way about Macron, the former banker, but as Macron made his money without proven impropriety, Bayrou has held his peace on that point.
If he gets in the race one can expect him to be more outspoken. His presence would of course plunge everything back into turmoil. He'd draw votes from Macron, and he'd draw votes from any Plan B successor to Fillon, or from Fillon himself if he stays in. It's hard to even guess at what the split might be in the middle of the spectrum. All bets would be off. In a 5- or 6-way contest with Bayrou in, Hamon could squeak by, or even Mélenchon. Macron is likely to have the advantage over Bayrou in the middle because he's been in the field for a while now and is a new face, but Bayrou would definitely hurt him. The waters would become incredibly muddied.
I think the odds are about 50-50 he goes for it.