Would the harry potter books count? The first book seems to be really light hearted to me, and things gradually get darker and more intense as you go on. Just my thoughts.
Demon Girl immediately comes to mind. I don't know about a subtle transition, it's more like misdirection? It starts out with a rather heavy atmosphere in a barren wasteland and then becomes fluffy reincarnation story, until you remember the main character is a demon and she murders countless adults in brutal fashion. It's a sort of mix of fluffy and dark -- while the MC maintains a saintly image among the people, she's in reality a demon in mind and soul and behind the scenes acts the part.
Shiro has this work in reading list. It is nice. But it doesn't have smooth transition at all. Tate no Yuusha was dropped by Shiro in gardening chapter.
how about Mimizuku to Yoru no Ou? it goes from dark to fluffy though. theeen, Akuyaku Tensei Dakedo Doushite Kou Natta. but this one swings between fluffy and dark a lot D: and Gong Hua, maybe http://www.novelupdates.com/series/gong-hua/ this one seems very merry and then there's betrayal and very serious stuff, but it's gradual. http://www.novelupdates.com/series/kyou-kara-ma-no-tsuku-jiyuugyou/ it has BL hints. thiiiiis one maybe. http://www.novelupdates.com/series/no-6/ it gradually goes from i'm happy to have met you to let's climb this pile of corpses. it also has BL hints, but nothing serious. and, in case you do decide you want to read something that jumps from fluffy to dark in one go, http://www.novelupdates.com/series/hitomishiri-onna-ga-isekai-de-seitenka-shita-kekka/
RRL Vocaloid thinks that novel is all over the place and not worth reading. RRL Vocaloid also thinks that the story is not fluffy at all.
Beseech the devil is kinda fluffy at the beginning and then gradually gets dark and then way dark Hmm could also perhaps add Heavenly Jewel Change OH and this one is good .. lots of green goblin fluffy at the beginning and then it falls to dark softly and slowly The Iron Teeth EDIT remembered another one How to avoid death on daily basis
Helck [Manga] the story pretty much a dumb thing until 50++ the story took turn and twist 1080 degree
Shiro is not that bad at math. 1080 is 3X 360 So story is still same. Shiro thanks you, but is the iron teeth is same as the one on Royal road? If so, Shiro did read it.
Yes its the same as on RRL .. the link I gave you was to authors own website. Where if you join with the patron club you can read ahead.
What is a Yandere? After All I'm a Lolicon Really Sorry Dungeon Defense I can't remember if DD is ever super fluffy, but there's some talk of it.
Shiro thinks that first two are too lewd for Shiro's tastes. Shiro will look into third one. Shiro did read first book, but it seems there is more now.
Dungeon Defense is quite dark in itself .. THO I have to say it has one of the best character interactions in a novel I have so far read. So for that it is already worth it to read. Tho I must warn you .. it is quite explicit .. gore and violence and sex is all not so fluffy but dark.. luckily it is only small part of the novel, but still if you can not stomach such things it is not a novel for you
Shiro doesn't mind gore or violence. Well, violence can sometimes become too much for Shiro, but that is extreme. Shiro will look into it and see, if third thing you mentioned is too bad or alright.
Idk what's more cringy and awkward, people who put desu~ at the end of every sentence or this 'thing' here.