In Season 3 of The Wheel of Time features some major deviations from Robert Jordan's original novels, particularly by killing og Siuan Sanche, which will have major ramifications for the story and other key characters.
In the book on which The Wheel of Time is based, Siuan is a central political player who survives till the end and shapes the White Tower's arc post-deposition. Therefore, her death on the show is significant and might lead to scarcely connected narrative points in the future. According to reports, this has been done to shift the focus to younger characters, and it suggests that Morgase might be next to see a similar fate, as The Wheel of Time is preparing Elayne for the throne. Read on to know in detail.
What happened with Siuan and Morgase in The Wheel of Time?

In Jordan’s books, Siuan is deposed and stilled but survives, becomes a rebel leader, and aids in uniting the White Tower. Her knowledge and manipulation of Tower politics are key to keeping Egwene’s rebellion viable. While in the show, killing Siuan cuts off the probabilities for these storylines. It seems like Egwene’s rise to Amyrlin may be entirely self-driven or supported by a different mentor.
In The Fires of Heaven, Queen Morgase falls under the influence of Rahvin, a Forsaken disguised as Lord Gaebril, who further manipulates her. She banishes people who are loyal to her and suffers alienation. Eventually, she realises she's being controlled and flees Caemlyn, becoming a political exile. She returns only later when Elayne is about to claim the throne.
What Siuan's death suggests about the character of Morgase in The Wheel of Time.

In The Wheel of Time, given how it featured Siuan's arc, it is highly likely that even Morgase's arc will be cut short, like how Olivia Williams only appears briefly in a flashback, not in current events. This suggests the show may use her backstory to establish Elayne’s political heritage, but won’t fully develop her own arc.
Since Season 4 of The Wheel of Time will adapt The Fires of Heaven, Morgase could be seen getting manipulated by Rahvin, and is later either killed by him or dies in the fallout of Rand's battle. Leading Elayne to become the queen without featuring any slow reclamation of power arc. This will also streamline some narrative, like how with Siuan's death, the entire hustle between the White Tower factions and the rebel Aes Sedai was removed, and with Morgase's death, Andor's politics could be simplified.
The show is indeed planning to cut short arcs that are unnecessary and part of the subplots, while focusing on the ones directly connected to Rand, Egwene, and other core protagonists.