Just to say that a few things mentioned here are already in game:

DoY as a high level hunting ground - it's already full of leveling areas with high level mobs, for the mission run around there.
I don't want to see the streets turned into this too, partly because I think that the complexity of the main zones could lead to safespot heaven, more importantly because I think that I would rather see the DoY faction zones become the long-discussed "super ops" than just another space with mobs spammed around.

Sympathy from killing high-level anarchy breed: Said high level AB, and missions to gain symp from them already exist - check out Tech Haven for them.

In general, I don't actually think that mob spawns need to be messed with that much. A few more decayed beasts and hazard worms would be good. What needs doing is a decent reward structure for what is already there. Back in the days of TFC, we would do things like go out and hunt Y-Replicants, or do some of the more unusual caves (and other, stranger things, like killing as many of the monk guards in the OZ-9 Church as possible). The thing is, the only reward for doing this was doing it - the XP and especially the loot was pretty dire. Make there be some purpose to killing Y-reps, surface DoY bots, etc and I think a lot will improve (that said, most players will still lack the interest to do anything other than run a prescribed "max efficiency" route through the game).