Just a brainstorming. I suggest that Gandalf could visit three settings during the season:
1) A tribe in Near Harad, which he has had dome contact with before. He could know some people of the oldest generation, or they could have died recently. He has traveled through Harondor and managed to avoid the patrolling troops there, maybe we want to show a little bit of that. The leaders of the tribe he is visiting aren't hostile or see him as an enemy, but they are proud and suspicious, they wonder about his motives, and don't necessarily listen to his advice. Gandalf learns that, as always, there is a power struggle going on among the tribes, but some of the leaders are arguing that they should unite. The united tribes would pose a great threat to Gondor, and the Haradrim could easily take over Ithilien, perhaps even pose a threat to Minas Tirith itself. This would give them glory. The tribe leader Gandalf visits isn't sure it is worth it though. Many men would lose their lives, and he wonders if someone has a hidden agenda. Perhaps there is a rumor that Sauron is about to return, which the tribe leader could have some interesting opinions about. Some people would want to see this leader as the leader of the union. If this is suggested, Gandalf will probably try to make him turn the offer down, or, should he accept becoming the leader of all of the tribes, that he should go for a peaceful policy, saying that establishing trade with the North would be a far better idea than war. Everything could be looking rather bright, actually, so far.
2) Another tribe, or Umbar. This is when everything goes wrong. A setting where Gandalf has to sneak around more, and risks being taken prisoner. He overhears conversations and learns of a rising force, a young Black Numenorean leader (future Mouth), and it is hinted that this young man is backed by a Nazgul. There's some kind of coup, the Black Numenorean takes over Umbar or a number of tribes. We understand that this new force will attack the tribe Gandalf has met earlier, and they will beat that tribe or force them to join the new union. If Gandalf has been able to convince the tribe leader to choose peace and trade, the Black Numenorean will send an assassin and have the leader killed, and take over the leadership of the Haradrim.
3) Gondor, Minas Tirith. Gandalf reports.