Two things you need to do to make slaves a challenge:

1. In the descr_campaign_ai_db.xml copy all decision entries from <faction_ai_label name="default"> (except the cases concerning slaves) into the section where it reads
Code:
	/////////////////////////////////////////////////////////////
	// SLAVE FACTION AI PARAMS.  THEY DON'T REALLY DO ANYTHING //
	/////////////////////////////////////////////////////////////
2. Edit the export_descr_buildings.txt so that slaves can build buildings (i.e. add slave into most of the buildings) and train troops (add slave into the units you want them to be able to build)