The Violent End of Duncan Malveine contains some novel elements: a randomized objective/clues, a collecting aspect, and guards that can switch between bow/sword at will. Of these three, the last one was probably the one that I liked the most, even though it's the most understated aspect of the mission. Why is that?

In my opinion, you'd rather have a mission that's unforgettable once, over a mission that lacks depth despite its randomized elements. As someone who reads detective novels, I was missing quite a few elements that could have made this a great detective mystery. There's no red herrings; every potential murderer has some reason to kill Duncan, and they're all equal. Of course, this has to be done so that the game can choose one of these people to be the actual murderer. In this sense, I believe the randomized mechanic holds back potential. Though the effort that has gone into this must have been insane, it's the result that counts for the ones actually playing. A system like this is cool, but had there been only one potential murderer, with a strongly written motive, lots of surrounding family drama and a bunch of convincing red herrings, this mission would've succeeded where it falls flat now.

SPOILER START
"My murderer" was Duncan's fantastic eldest daughter. She, a devout fan of Hammerite religion, was upset because her father had converted to the Mechanists. She had poisoned him because of this. That's it. It's as surface-level as you can get. Someone on TTLG mentioned this being "one was the hardest for me". It didn't feel hard. There was a vial of poison standing on her nightstand. The crime scene report already mentions poisoning, and the library gives you the very obvious final clue. Again, the fact that this changes between playthroughs is amazing on paper. But in practice, it just means having a slightly different shallow mystery each time.
SPOILER END

I just expected more on this front, something that could have achieved without the random elements.

The collection aspect takes a lot of time, and the reward is small. The collecting does introduce you to some genuine jumpscares, though, so be careful of that. Though most of the collecting is busywork and not that fun to do, it's a fun side objective that's entirely optional, and gives you cool backstory on the final days of Duncan's father (sadly entirely disconnected from the detective mystery).

Now it's time to leave behind the raw criticism, and judge the mission by what most Thief missions are about: stealth, atmosphere, and level design. The mission absolutely succeeds here. A very solid (and huge, probably the biggest in Thief period) mansion with a cool Watcher system (you can't just turn them all off) and the aforementioned "flexible guards" make for a really enjoyable experience. Though the mansion is a little on the predictable side (everything is smoothly mapped out, and the mansion itself is very straightforward), it has some really cool details to it. Its bright, opulent design is a welcome change after having played a bunch of gothic, run-down places.

Play it, it's great. Just don't expect too much out of its selling point.

"The Violent End of Duncan Malveine contains some novel elements: a randomized objective/clues, a collecting aspect, and guards that can switch between bow/sword at will. Of these three, the last one was probably the one that I liked the most, even though it's the most understated aspect of the mission. Why is that?"

Probably because either - people K.O. everything there is, or they ghost. In neither case, people will even make acquaintance with the guards' weapons.

BTW, I think you're being a tad unfair with the "predictable" part. I think this mission's gameplay and objectives are pretty unique. At least I haven't come across much "predictable" stuff.

The detective aspect is unique, but it doesn't add much. I was actually on the fence on calling it "predictable" because even though I really believe there's a part of it that feels that way, it doesn't really describe the mission as a whole. It's mostly the level design being very straightforward that gave me the idea to name it that.

Honestly, it's more a criticism of the detective element. Even without that element, this mission would still be very solid, if a little straightforward in terms of navigation.