The initial promo of Sona walking away from Dev at the cafe, has already happened. Although they promoted a brief walk away what we got was how she walked away at the hospital.
After the engagement and few other incidents, Ritwik chapter will end (hopefully amicably) I am guessing Dev, Sona and Ritwik will come to know that Ritwik was brought into the picture by Ishwari.
Dev will not be able to agree to marry Sona. This brings us to promos 2 and 3, where they do get on with life, Dev finds excuses to meet Sona and she continues to make diet charts/advise for Ishwari.
Where they go from here is for us all to wait and watch.
Personally, I think it is a huge challenge for them to remain in love with Ishwari as a barrier but then not every love ends in marriage and neither does it have to end in marriage to have a happily ever after.
I look forward to following their story, absolutely unique but a beautiful love story, nevertheless.