Tweak Obsidian crafting to accept 5* precursor gear - that is either 4* or 5*.
Notice that Obsidian stuff consumes twice as much material as a typical 5* craft? It seems like a missed opportunity to extend the crafting system just a bit more, presumably since there will not be any 6* and beyond then this would be the closest that we'll ever get.

One issue that I can think of however is that if people convert their current 5* to obsidian, then that could be lost crafting revenue for the lower star crafts if they choose not to replace the consumed item in the process... But since the Obsidian craft requires 2x material costs, then why not consume 6 5*orbs?

Huge errors in this plan? Maybe. Just a solution to a problem (for me anyway :P)