Below are paragraphs from Chronicles vol.3 regarding the Siege of Dalaran during the Third War, with some highlights of events that weren't originally too obvious.
The Siege of Dalaran details
Tichondrius came into possession of the Skull of Gul'dan from the same vault where the Book of Medivh was kept in those same moments, therefore
without making the dreadlord playable (must be excluded for game mechanics reasons, Tichondrius has divine armor), the inclusion of the Skull of Gul'dan could have been inserted into the background of the final scene of the siege, with Arthas watching Tichondrius claiming the skull and then rushing Kel'thuzad to take Medivh's boo
k by saying his regular line of dialogue
"The spellbook is all yours, lich. Let's take it and leave before the wizards amass for their final attack", with this small canon-friendly inclusion we make known both how the Legion came into possession of the Skull and how Arthas knows that it is being used by Tichondrius, preparatory elements to later chapters which form a good narrative bridge.
Additional note: from the Arthas novel we learn that Sylvanas was present during the siege of Dalaran, however it seems she was not present at the boss fight with
Antonidas.
Under the Burning Sky details
According to Chronicles, shortly before Archimonde's summoning, some of his lieutenants were summoned: Mannoroth and Kazzak.
Mannoroth appearing now feeds very well into the following orc campaign, which starts from the chapter immediately after this one, and a single voice line from WoW could fit him in nicely (see attached .mp3 file).
I wouldn't make Mannoroth playable, it would be just a cameo shortly before the end of the timer.
Kazzak is a doomlord, a very powerful type of doomguard, his character was originally absent from Warcraft 3, he later gains importance for being the last surviving high lieutenant of the Legion in Azeroth and the main responsible for reopening the Dark Portal in WoW and thus the for TBC expansion. However, his role during the Third War is marginal,
so he may or may not be included, if he was included, it wouldn't be bad to see him (a very little) later during the Night Elf campaign like when Tyrande is escaping from doomguards in the second nelf chapter or in the chapter set in Winterspring with unreacheable enemy bases. He is also voiced in WoW with many voice lines that could be integrated (see attached .mp3 file)