- What exactly do we mean by diversity?
I would look at this almost entirely from a jurisdictional and ISP level. I believe the biggest "sudden impact" threats to the tor network are going to be from legal changes (jurisdictional, i.e. "save the children, nullroute the nodes") and local business policy changes ("sorry tor customers, no more tor egress from our DC due to complaints").
I'm not sure which thread I mentioned this on so I'll put it here to be sure. I think one main thing needed is a project to catalog all the current exits as to their diversity... Box: ISP/hoster, AS, datacenter, country, upstream AS/Tier-n path, relay-operator Relay-operator: country
Without that, seems like placing nodes amounts to, 'Well, we don't have any in Iran, let's go there'. If it turns out that IP is more or less fed as a courtesy from UAE across the gulf, there's not much gain. Repeat analysis for any of the above parameters.
More nodes are probably good, just not all as USA, Equinix, Level3, with whatever hoster has a rack in all the DC's.