Question: What would it take for an Evolutionist to believe in Creation?
First, it depends a little on what you mean by 'Creation'. An 'evolutionist' can very well believe in a Creator.
But - if evolution was falsified by DNA sequencing not showing evidence of common descent, if the fossil record didn't show evidence of gradually progressive complexity in species through time (that's a simplified statement, but accurate enough here), if animal species were scattered 'helter skelter' in the geologic column like rabbits found with dinosaurs, birds found before reptiles etc., if all dating methods didn't show evidence of a really old earth and universe, if there were no atavisms ("throwbacks"), no vestigial features, no signs of retroviral insertion points in DNA that must have occurred in a shared ancestor... To put it short, if the theory of evolution did not describe what we see in nature, then it would be falsified, and we'd have to look elsewhere for an explanation. That in and of itself would not mean everyone would have jumped on a literal reading of the Bible, but it would perhaps have become a story that was easier to swallow scientifically.
As natural science cannot take invisible, unprovable entities into account for natural phenomena, it can only describe what we find evidence for in nature. If we thereby actually end up describing God's handiwork without directly referring to that God, that's just the way science has to be, as it has to be evidence-driven.
Evolution theory was not created as an idea first, then attempted to be supported by evidence. Yes, many will say that some sort of evolutionary thought has been around since the ancient Greeks, but it didn't become a natural science until we (creationist naturalists, in fact, as there was no real alternative at the time) found "strange things" in geology and biology that didn't really fit with - for example - a literal interpretation of Genesis. Rock was older, and the layers were filled with species that no longer existed. As signs pointing to a gradual formation of species came about, the theory was fleshed out. Darwin's grandfather, although a creationist, wrote a poem I think it was, detailing evolutionary thought. But it was his grandson (and someone else at the same time, Alfred Wallace) that came up with the mechanism with which it would work. Not the beginning of life, but the ever changing property of life as seen in nature. Since then, a lot has happened, just as a lot has happened in cell- and germ theory since their inception, following further research and findings.