I used to be 100% behind this theory believing that Shanks will try to stop Blackbeard but fail and end up dying, which will in turn give Luffy motivation for fighting Blackbeard........but, currently I've realized that this will most likely not happen.
Shanks is being made to be this guardian/gate keeper figure of the One piece, that does not directly interfere with the PK contenders. If Shanks wanted he could've made Luffy's journey easier or taken Blackbeard out much earlier, but he hasn't and doesn't seem interested in doing so. He now wants to go after the One piece, but in truth he will just be the last stop for whoever comes for it (i.e. Luffy).
The one Blackbeard is most likely to fight is Luffy on the path to PK, which will parallel Roger vs Xebec, two Ds with opposing views of freedom fighting it out. The winner will then head on to confront Shanks prior to Imu. Another parallel between Luffy and Blackbeard is that they both left their mark on Shanks: The scar and missing arm.