Nah man, you can't give someone who plays in the Netherlands a Ballon D'or podium finish unless he does something ridiculous in the UCL. That's pretty much the unwritten rule. Ziyech is no threat to Ballon D'or contenders, currently, unless he leads Ajax to another Cinderlla run, while standing out as the clear star (unlike last time, where Ajax had many players playing very well).
That won't be enough though, that's my point. If Ziyech gets 30 assists, but Tadic and Promes and de Beek all play very well (which they'd have to, if they're going to make a deep run in the UCL) then they'd split the votes. There is no limit to how many votes a club like Barcelona can get. They can and have got 3 people on the podium. A club like Ajax will only win so many votes. If there's just one person standing out above the rest, then that limited amount of votes could be enough to get the star player on the podium. However, if Ajax have many players who play well, then their limited votes will split and none of them will make the podium. Look at the UEFA PotY voting for 2019. De Ligt and De Jong were Ajax top two, with exactly 27 votes each, losing out to Hazard 38, whose team received less votes than Ajax, but Hazard took all the votes for Chelsea.
Which is why I say results, CL topscorer titles or goals don't even matter. De Vecchi for the 1920s is the same story. The big cultures agenda (the same FIFA cartel that gives special privileges to each other) is so strong that he is ahead regardless of consistently worse results (consistent podium results vs a sixth place as best result).
I think the only way it could work is if Ajax has a really marketable player (not sute Ziyech is one) who is also a forward who is the league top scorer, and also leads Ajax to the UCL title (must also score in the final). It's only a ridiculous season like that which I can see an Ajax player winning the Ballon D'or in the modern day.
Bergkamp came close playing in the uefa cup (losing to Roberto Baggio who won largely due to his UEFA cup performances) I get you said modern times and ziyech isn't exactly marketable Still I reckon you are overlooking the power of raw statistics 30 assists in a single club campaign has only been done once in the 21st century KDB,Messi and ozil all came extremely close but not quite there If a relatively large proportion come in the CL and he reaches a SF for the second consecutive season Even in messi's best season 2014/15 iniesta was voted MOTM in the final(officialy i mean ,not getting into whether it was deserved or not) My memory could be a bit sketchy here but wasn't iniesta also MOTM in the QF vs psg for his dribbling run assist on neymar I just don't think ziyech would have to be the outright best in every single KO match It's enough to be one of the best and not a 'passenger'