Our immediate diplomatic concerns will be primarily integrating the Lordaeron holdouts of LHC, Hearthglen, and eventually the ones in Southern Lordaeron into the Scarlet Crusade.
While we're doing this contacting the High Elves at Quel'Lithien Lodge to cooperate against the Scourge, and offering to take in any High Elf refugees which they're currently sheltering assuming they've already split into High/Blood Elves. Because they've a very limited ability to grow food whereas Tyr's Hand can plenty of room to grow food.
Eventually I'd like to bring them entirely into the Scarlet Crusade(Rangers are excellent guerrilla warfare soldiers something we lack). At a minimum we'd need to demonstrate that we're willing to take steps to insure their culture is preserved, and we won't be even remotely similar to Garithos.
There are two things the Blood Elves can do for us which would save a lot of lives in the long-term. Assuming the Blood/High Elf racial split hasn't occurred yet give the High Elves a one way trip to Tyr's Hand via a boat instead of kicking them out into the Plagueland to fend for themselves.Other than that cooperating with us to clear the Ghostland, and eventually the entire Eastern Plaguelands, because we will be able to provide the necessary manpower to clear their land, whereas they can sabotage Scourge engines of war, and have a considerably larger pool of mages to counter necromancers.
To do anything with Tarren's Mill, or Southshore we'd either need to purge Anderhal before conquering Strahnbrad(alternatively charm them into joining us), sneak a large enough group of soldiers past Andherhal to conquer Strahnbrad to use as a base, or contact Aerie Peak via boat to work out a push through Durnholde Keep to establish a secure line of communication with Southshore, which gives us a reasonable method for contacting Stormwind to ask for help with our Undead problem.
The third option has the advantage that it would allow Alliance forces to clear out Tarren's Mill, and Alterac before conducting a three way assault on Anderhal from Strahnbrad, Hearthglen, and Tyr's Hand.
Since we'll have already damaged the Syndicate by taking Durnhold Keep finishing the job, and tipping the conflict in the Arathi Highlands in favor of the The League of Arathor, and Gallen Trollbane seems like a course of action which would gives us an ally with excellent soldiers.
Also is anyone else amused by the fact that should we choose to join the Alliance Angelica has fairly good odds of being one of the more influential leaders, in part due to her high diplomacy stat?