Modric is not losing the Ballon D’or if he plays well and Croatia wins the World Cup. Best midfielder in the world with a Champions League title and a World Cup win? Being a key player for both?
Now whether Croatia wins is another matter.
If Croatia win it will go to Modric but a real long shot seeing as they've never reached a final.
If Brazil win then Coutinho/Neymar have a slight chance but neither have done much for club (Coutinho was cup tied for CL and Neymar injured most of the year). If Coutinho had stayed at Liverpool and won the CL, or at least the final, he'd have a better chance.
If France win it then Griezmann or Mbappe have a chance, particularly if either Zidane 98 the final.
If England win it then Harry Kane has a chance.
If Spain win it then I can't see one player making the decisive contribution, Costa isn't winning a ballon d'or for example. Possibly Iniesta and that'd be more as recognition for his career. He still didn't win it in 2010 despite scoring the winner in the final.
So it depends which team does win it. Ronaldo still likely.
I think Messi has won his last Ballon D'or anyway unless Barca win a CL the next couple of seasons. I think this year will be Ronaldo's last Ballon D'or unless Real win CL again next season.