Whether their feud and match is worthy of being on the WM30 card comes down to whatever else of importance ends up on the card as well. I don't see how a Cody/Goldust feud wouldn't be worthy of earning a spot somewhere (they're currently apart of the main storyline, after all), especially given that Dusty Rhodes will certainly get involved in the brotherly feud somehow and WWE likely wants as many legends as possible to play some type of role (major or minor) in next year's Mania. Dusty's two sons fighting it out gives him a purpose to be there on-screen.
Here's one way to book it:
Since Cody and Goldust are on a roll (beating The Shield at Battleground and giving Harper and Rowan their first ever loss in a very good match on Smackdown last night), I think they should be the ones to defeat Rollins and Reigns for the Tag Team Championships at Hell In A Cell. Then prior to the Rumble or on the card itself, they lose the belts to Rowan and Harper and this causes tension between them because of a screw up that one or the other makes (i.e Rhodes accidentally hitting The Disaster Kick on Goldust and causing him to get pinned.) Their tension escalates even further in the Rumble match later on, with someone trying to get Goldust over the top rope and then Cody comes up behind and lifts them both over the top rope, eliminating his brother along with whoever else was trying to eliminate him. Similar to the way Hogan eliminated Savage in '89, which caused Savage to blow a gasket. This pisses Goldust off and really starts to pit them against each other.
This leads to the Elimination Chamber PPV, which shows one of two things happening: either Cody wins his qualifying match for the chamber and Goldust doesn't and so Goldust distracts Cody during the chamber match and causes him to get eliminated (or slips into the ring when the chamber door is wide open to let a recently eliminated participant out, and hits the Curtain Call or Final Cut on Cody) or they both qualify for the chamber match. And it comes down to the two of them and whoever the WHC is. Goldust gets eliminated and then immediately afterwards, Cody gets the upper hand on the champion and right when it looks like he has the championship won, Goldust (who is only just now recovering and hasn't left the ring yet) hits the Curtain Call or Final Cut on him. Maybe even Shattered Dreams on top of it. And he causes Cody to lose the match and have the title slip from his fingers just like that. Easy to build to Wrestlemania from there.