If you have some idea of your plot by now, get out whatever plot notes you have and write down ten things (minimum) that your speculative element can do to your plot.
If you don't have any ideas about your plot, but you do know what kind of speculative elements you want in your novel, that's fine. Just look at your notes for that element and start asking yourself "what could I do with this element? Can I have my hero or villain use it? Refrain from using it? Is it a threat? A convenience? Both? What happens when it fails?"
My plot: Boy meets girl, boy and girl fall in love, boy loses girl.
Ten things my speculative element (Transhumanism) can do to my plot:
1. The exotic Transhumans can turn the boy's eye in erotic ways, causing him to stray. Can they lead him away from the Super who loves (?) him?
2. Why would a Super who can live for so long want to be pair bonded with a human? What sort of life could they make with each other? Will she, can she love him for the short time they can be together?
3. What would attract a Super to a human? They are small, weak, ugly, stupid.
4. How does society see them together? Are they disgusted by seeing them together? Are they outraged that a mere human dare be with a Super, or are they incensed that a Super debased herself with a human?
5. What struggles does the Super have to overcome that she can get help with from a human?
6. Why would a FOREIGN human be better for a Super than a Howler human?
7. Are they sexually compatible? Will their parts fit together? Is she so different that she is a different species and cannot reproduce with him?
8. What is love? What is the nature of their love? Can they love each other equally? Can they even love each other in the same way?
9. Does her brain work in such a radically different way that they can't understand each other at all? If not then what really is the basis of their relationship? Would he just be like a pet to him? Can he live with that? Can she?
10. I want a rescue, I want him to rescue her. How can he do that when she is so far superior to him?