Apple SIM + 1 physical SIM tray? Maybe. They already did that in the iPad Pro 9.7".
2 physical SIM trays? Won't believe it 'til I see it.
That said, Dual SIM seems much more likely to me than expandable storage.
Moving one tier up from the lowest storage size costs only about 15% of the phone's price (unsubsidized). Or, to put it in absolute terms, only a 100$. Not really enough to make the difference between a state-of-the-art/high-end phone and an older/mid-tier smartphone elsewhere. Most customers are just going to bite the bullet and pay the difference rather than foregoing the iPhone purchase entirely. I'm sure it's greatly improving Apple's bottom line.
On the other hand, I doubt there are many customers who buy a second phone just because the need a second SIM slot. A second SIM slot might entice buyers being on the fence between choosing an Apple or an Android handset, especially in Asian markets. I wouldn't be surprised if additional sales due to having Dual SIM outnumbered the loss of sales due to users just getting it as a second device.
Might also help to better tie in customers into Apple's ecosystem, if they carried just one (i)phone - rather than having a secondary Android device.