Rather than adding more Cenedril instances, how about a reduced difficulty on existing zone instances that allows solo runs of things like ToSH, GC, actually all of them so they can be soloed. One player, solo mode. It would need to be tough enough that you still need well-built gear to complete it. It would have the same TP per mob kill that each member of a party of 12 gets from hard mode. Since you wouldn't be sharing loot with the party, it would need to drop the purple set gear from the bosses one tenth as often as hard mode, so it doesn't become a gear farm. It would, however let you run it and thereby get gear over time. The basic set up would be 4 levels, hard, normal, easy, and solo.
In Solo mode you need to be able to run it without strategies that require multiple players. That means HoS, for example would need a revamp of the colors boss, but ONLY in solo mode. The other modes would still need 5 players.