I've seen this question pop up a lot, and the usual advice is to subvert tropes or make the characters hate each other. That can become its own cliché, though. For me, the freshness comes from redefining what 'burn' even means. It’s not just about delaying a kiss. It’s about building a shared language that only those two characters could develop, where the intimacy is in their private jokes, the way they hand each other tools, or a specific sigh they both understand.
Instead of external obstacles keeping them apart, try internal ones that are genuinely hard to solve. Not 'I have a dark past,' but 'my life goals are fundamentally incompatible with a partner, and loving you forces me to question my entire identity.' The slowness is in the painful, careful negotiation of that. Look at 'The Remains of the Day'—it’s a masterpiece of slow burn because the romance is in everything left unsaid, every duty chosen over desire. The tension isn't will-they-won't-they; it's a tragic study of character. The ending feels fresh because it’s devastatingly honest, not neatly packaged.
Give them a rich, co-existing world outside of their attraction. Let them be competent at things unrelated to the plot. The reader should believe these are whole people who would exist without the romance, which makes the eventual coming together feel earned, not inevitable.