Inspired by this question: Would allowing the Rogue sub-class (Assassin) to benefit from Supreme Sneak be unbalanced?
The Assassin roguish archetype is considered by some to have very weak class features at levels 9 and 13. The linked question suggests replacing the Infiltration Expert feature with the Thief rogue's Supreme Sneak feature, which I think fits thematically and mechanically, although a sneaky rogue with proficiency and Expertise in Stealth probably won't really need it or notice the benefits that often.
In a campaign where the Assassin's RAW features at levels 9 and 13 won't really come into play, I like the idea of replacing one of them with Supreme Sneak as the linked question suggests, but that got me thinking: what about the 13th level feature, Imposter?
For reference, the Imposter feature description (PHB, p. 97):
At 13th level, you gain the ability to unerringly mimic another person's speech, writing, and behavior. You must spend at least three hours studying these three components of the person's behavior, listening to speech, examining handwriting, and observing mannerisms.
Your ruse is indiscernible to the casual observer. If a wary creature suspects something is amiss, you have advantage on any Charisma (Deception) check you make to avoid detection.
Naturally, the first place I looked to was the 13th level feature of the Thief rogue, but I don't think Use Magic Device is appropriate because it doesn't fit with the theme of being an assassin, and it's also a defining feature of the Thief archetype, hardly the same as a feature about sneaking, which by comparison is quite generic.
So, I considered the other roguish archetypes, looking for something that seemed thematically appropriate, and found the Scout's 13th level feature, Ambush Master. It sounds like something that fits the flavour of a stealthy assassin, without being too niche like Imposter.
The description of the Ambush Master feature (XGtE, p. 47):
Starting at 13th level, you excel at leading ambushes and acting first in a fight. You have advantage on initiative rolls. In addition, the first creature you hit during the first round of a combat becomes easier for you and others to strike; attack rolls against that target have advantage until the start of your next turn.
This feature seems to synergise well with Assassinate, especially the advantage to initiative rolls... but maybe it synergises with it a bit too well? It's certainly a lot more combat oriented, and clearly makes the Assassin archetype stronger than RAW, but does it make it too much stronger?
Would swapping the Assassin's Imposter feature for the Scout's Ambush Master feature cause any major balance issues for an Assassin?