With Germany and the Netherlands now having most of their parks open, Polish parks opening this week and Scandinavian parks reopening throughout the month, two big countries that are still yet to reopen parks are Belgium and France.
Parc Asterix are set to reopen on June 15th, with a max of 5000 people a day. Haven't looked into other French parks in much depth, but with the exception of Disney (which I believe is classed as within the Paris region, which is still hit quite hard?), I believe other parks are reopening soon too.
As for Belgium, the story is a bit more interesting. The current date set Belgian government was June 8th, which is set for review this week. The current plan however is that this date could be pushed back until July 1st or August 1st, to which all the big Belgian parks have reacted...in an upset manner... https://www.looopings.nl/weblog/14563/Belgie-wil-pretparken-tot-1-augustus-gesloten-houden-attractieparken-reageren-woest.html
With Belgian people able to cross borders to Netherlands and Germany soon (if not, already), they're particularly worried that when they do reopen, they'll have lost the reopening boost that would come, along with the fact they miss half of the summer season.