It should be quite unpredictable with 6+ spots on offer in each event but I'll give it a go.
Dressage - +
Top 6 countries seem to be the firm favorites at the moment, however Spain will definitely look to steal a spot, mainly from the Netherlands I suspect. Portugal and Russia meanwhile could get in if a lot of things go their way. Australia shall get the Group G spot by default.
Eventing -
This one is already less predictable. Great Britain is likely the biggest favorite for qualification (they have ridiculous depth). Australia, Germany and New Zealand should do alright. Sweden and USA are probably the most vulnerable ones on this list. Canada, France and Ireland could be close, followed by Belgium and Italy. Canada could do very well at their home continent (already proved it in the past by taking shocking team medals both times the USA hosted eventing worlds). French selectors seemingly learned nothing from Strzegom disaster last year. Not worthy of Olympic champions, I can see them getting elmiinated again. Ireland probably has their best team in years on the other hand. Belgium/Italy are more on the mediocre side. I suspect at least one surprise qualifier should get in thanks to avoiding major XC penalties.
Jumping -
I'm quite confident for Belgium, Ireland, Netherlands and USA. Germany and Sweden not so much. Italy and Switzerland could be very close to taking a spot as well. Brazil (not consistent enough), Canada (lacking big names), France (a lot of unfamiliar names), Great Britain (inexperienced), Spain (inconsistent) will be looking for spoilers. And finally, I can also see Mexico, Egypt, New Zealand or even Israel creating a major surprise.