As for Fractal Trees. I found out about those and send a mail to phoronix since those things really seem to be scaling very well! The downside of those things is CPU bound. And they will certainly use your CPU. Perhaps even too much and that might be a potential issue.
As for Fractal Trees being a drop in replacement for B-Trees.. Perhaps, don't know. Yet again, remember very carefullt that TukoDB (that uses Fractal trees) is a drop in replacement for InnoDB! That last part is very important, InnoDB!
simply "replace" a current dataset build with B-Tree with a Fractal Tree is probably not very likely to happen since they likely store data different.