Since the mod is set in Bilbo Baggins' birthday (3011, it is 7 years before Saruman actively turn against the free people (3018), and by that time he also didn't anger the Fangorn at all
And even after that, his goal is always simple and clear: obtain the ring, control it and use it overthrown Sauron
His military campaign is always towards the fellowship of the ring (where he think the ring is), besides that he never actively attack a free people faction, rather seeks more to control other factions (agents in Shire and Bree, corrupt Rohan king) than to conquer.
With this quote it also says about his motives are at least "good" to himself:
In the chapter "the White Rider" (LOTR Book 3, chapter 5), shortly after Ugluk's foray has been defeated, Gandalf gives a detailed analysis of Saruman's motives and Sauron's concerns. He says: "Even reckoned as a lord and captain Saruman has grown very strong... Saruman also had a mind to capture the Ring for himself... [Sauron] now has Isengard to fear as well as Minas Tirith..."
And the fact Gandolf and the rest of ppl try to parley with him even after his defeat says that they realize his intentions are not entirely evil, rather a different route of "good".
To summerize:
1. All good factions treating Isengard as "mortal enemies" like all the other evil factions is overdone, especially since the timeline is set in the birthday
2. When the major quests come, Isengard should be the most versatile faction with diplomatic relationship (along with Dunland), because in the books all his military campaign is to take the ring for himself, whichever faction control the ring is sure to be his enemy, but other than that he shouldn't be an active enemy (Tho I know major quests are long way to come..)
3. Would be nice if we can have his agents in the game, like the half-orcs and agents in Bree
4. Since half-trolls' lore are mostly mod-cannon, may i dare suggest that they should be aligned with Isengard (forming a "third side" along with Isengard and Dunland, tho they are probably still friendly with Mordor since they appeared in the Battle of the Pelennor Fields), afterall there are plenty of allies of Mordor already, and with the mod-cannon lore, they were abandoned by Sauron, it makes more sense for Saruman to contact them first