I'm going to start running Lost Mines soon, with the idea of moving on to PoTA.  It looks like a few tweaks will mesh the adventures pretty well.  Here what I've been thinking:

• Vizeran DeVir used the Spellforge to craft the four elemental weapons - this is what burned it out.  The Black Spider is his apprentice, left to supervise Wave Echo cave after his departure.  The magic items found at the forge will be replaced with elemental-type weapons (maybe a Flametongue and a Frostbrand).  Vizeran left the gazer there, so I guess that will remove the option of the players talking their way past the thing
• The dragon at Thundertree will be replaced by a red dragon being courted by Fire Cultists.  The tower will be replaced with a volcanic vent left over from the destruction of Thundertree.  The higher CR of a young red dragon gives me some heartburn, though.  I suppose I could make it a black dragon being courted by Earth Cultists, but that's a less direct elemental link.
• I'll move some of the sidequests from PoTA to the Phandalin area.  I was thinking of swapping Lance Rock for the Old Owl Well encounter, and modding the "New Management" sidequest to be the Miner's Exchange in Phandalin instead of the Bargewright Inn.

Any other thoughts or comments on meshing these two?

Maybe it's a blue dragon being courted by Air Cultists?