OOOOOH okay, what if i give you more than one... gasps. ok for rock au a lot of this might spoil an upcoming fic but whatever we roll
rock au - 1) i like to think that whenever ivan's being particularly annoying till likes to push him against a wall (ivan just lets this happen because he's into being manhandled trust me) and make out very aggressively with him and god does ivan have an obnoxiously wide smile the entire time (prompting till to get even worse (better))... they leave with till's tie askew and beanie in hand along with ivan's hair mussed and lips bright red and wet. yeah
2) big fan of till hating rainy nights in any au, but in rock au in specific, i'd like to think that ivan left the band (temporarily) on a rainy night, prompting till to refuse to leave him apartment on one such day.
3) till despising ivan's aegyo, slapping his hand down, hissing at his ear, "if you wanted to do this cutesy bullshit so bad you should've debuted like the goddamn idol like you were supposed to be," and ivan would pout, "aw but till, you'd miss me too much." and till would glower despite his ears burning red, unable to deny him.
cafe au - YALL KNOW HOW NEAR AND DEAR CAFE AU IS TO MY HEART :(( i still have a few ideas concerning it actually!
a) i'dl ike to think till and ivan purposely take more closing shifts so they get to make out atop tables after (fulfilling some weird exibition kink ivan has and till likes to pretend that he doesn't have)
b) they switch. a lot. like a lot. i think cafe au is /the/ most switch au i can think of actually. till's sinful bow tying his apron tgt, ivan's stupid fucking rectangular glasses, buttoned-up accountant looking ass, begging to be messed up... so much fun to be had with these mundane freaks
c) BUT THEY'RE ALSO!! SO FLUFF!! BECAUSE ITS A CAFE... because till gets stupidly jealous, because ivan steals one of the weekly specials from the kitchen counter before issac gets the chance to put them on the display rack and till indulges him by distracting issac with, 'hey dad-I MEAN ISSAC do you wanna listen to my sick beats' (i'd like to think till sees issac as some dad figure LMAO)

it's been 5 days. but here we go. i'm ready to give you my definitive answer to this question anon. buckle up.
luka - flan - as you said, it's extremely fitting, not just aesthetically, but also symbolically. flan is typically baked atop a waterbath, a very delicate process, and when ur inexperienced, you tend to fail a lot before you make the most perfect, smooth-topped product, devoid of trypophobia-inducing holes. very fitting, wouldn't you say? on top of that, the flavour is subtle, glides easily, and is very palatable, adaptable to many tastes, you don't often meet people who wouldn't go for a perfect flan.
ivan - red velvet cake w/cream cheese frosting - i was debating between black forest cake and this at FIRST, but i thought about it further, and cream cheese frosted tang amidst the general sweetness of red velvet, chocolate but 'refined' so to speak, appearing more fancy, more elegant, despite its common roots. i think it's cool how red velvet can have either vanilla, buttercream, or cream cheese frosting, and i think the shock factor of getting it with cream cheese suits ivan quite well, you get a contrast despite what seemingly looks uniform in flavour, texture, and appearance. (also red velvet cake generally being more crumbly, fragile despite what it'd seem.. yet still retaining a rich chocolatey flavour... heh)
sua - panna cotta - made with humble ingredients, much like flan, is smooth, easily palatable, and generally likeable due to it's subtle flavour. it's malleable, as you can add in a varied range of fix-ins, making it customizable, something anyone would enjoy with the right tweaking. it's not too loud either, it doesn't demand ur attention, but it draws you to it nonetheless. also insert waterbath logic with the flan.
hyuna - rum cake - despite the obvious alcoholic reference lmfao, i think rum cake is associated with the hearth of the holidays, and is seen as a comforting dessert that many would enjoy due to its rambunctious taste, serving well for one's psyche amid the cold nights. it's such a treat, soaked and glazed, topped with walnuts for a beautiful crunch. it's so hyuna to me, idk, warm, inviting, full of life and heart, star potential.
till - fruit tart (lmfao) - i was debating between this and mint chip bc mint chip works as well but felt too obvious LMFAO. i think fruit tart can seem a little intimidating, due to pie crusts and custard being generally more challenging to bake, but boy are they worth the fucking risk. you've got the crisp, yet crumbly crust, paired with the not-too-sweet, rich custard, which you need to temper JUST right, it's a consistent process of cooling and steaming and contains multitudes to it, which can make it seem really difficult to control. the topped mixed fruit on top, all providing an adequate bite, tang and texture to its overal flavour. each bite's a new experience! some good bites, some bad, depends on what fruits you like really, heh, very mixed bag, but an overall favourite, till coded fr.
mizi - battenberg cake - i think mizi's was the hardest for me to decide because i wanted something that generally seemed very pretty looking, and something that had like, a multi layered context. battenburg cake is a checkered yellow and pink cake, layered in marzipan (hope) and held tgt with jam (love, bleeding), it was originally made to appease to the british pallate while retaining german aesthetics. mizi appeasing to both her owners and her peers due to her perceived innocence, her beauty. it celebrated a marriage, a matrimony of love.

anon this has me wondering if i needed to get an audhd diagnosis for the 122134th time HELP ME I DIDN'T REALISE THIS WAS A SENSORY ISSUE SOMEHOW?? BUT YES HOLY HELL TILL WANTING TO CARRY ON WITH PHYSICAL ATTRIBUTES TO IVAN'S PRESENCE... WHAT A GORGEOUS NOTION, ah i need to stop capsing. VERY CUTE. i like ivan taking till's clothes asw bc a) it's menace behaviour and b) 70% of till's closet is oversized ANYWAY so it would fit ivan quite well. i can imagine till walking in noticing ivan wear his emo sweater or whatever and going 'when the hell did you even TAKE THAT' and ivan shrugs, 'if you didn't notice it doesn't matter' and till punts him into oblivion yes i love this