I'm playing as a custom race using the Altarian tech tree, but I expect the same problems could be found playing as Altarian.

The Entertainment Center does not upgrade to Stadium, and there appears no way to build a Stadium.  Instead, there are Healing Pools (stolen from the Torians in GC2 if I remember correctly) that upgrade to Meditation Centers.  So if you build an Entertainment Center, there's no upgrade path, since it won't upgrade to either a Stadium or a Healing Pool.

Posting because I don't see this on Seilore's list.