Some gaps I think are interesting:
0234 and 0256
This could be a sign of Oda having future plans for the mysterious St Briss (the ship that fell from the sky). Flipping through the relevant chapters, I can't see any living characters who deserve a spot between Dr Potsun and Masira, or between Masira and Roshio. If they were random members of Masira's crew or any of the sea monsters seen underwater there'd be no reason to hold them back.
But then we have the skeleton that scares Usopp and the woman in the picture Sanji looks at. Could there be a backstory to come for this pair in a later chapter?
0256 - 0260
The Five Elders, obviously. But will we finally be getting their cards perhaps sometime this year?
0353
Almost certainly Goro from Gedatsu's cover story, but why wasn't he included with the previous sets of EX-cards?
0622 - 0626
Seem to be left for the Marines' Giant Squadron. We might be due to learn more about these guys on Elbaf.
Lots of small Wano openings
There's going to be a separate lot of EX-cards just for Kaido's Gifters, I'm sure of it.
1394 - 1497
Over 100 empty spaces between the introduction of the Mimawarigumi (chapter 951) and the introduction of Oden (chapter 960). Are we sure there were 10 new characters per chapter for that stretch, or are some background characters from either the flashback or Onigashima being pulled forward?
1508 - 1551
The space between Toki (chapter 964) and Ulti (chapter 978) is pretty extreme too. Even naming every single Gifter and tracking down panels for all the SBS-named Roger and Kid crewmembers from around that time feels like it would struggle to fill it.
We know Oda's involvement in the databook is largely indirect and editorial, so maybe it's a stretch to guess the St. Briss corpses and Giant Squad were held back deliberately for future plans. But it's certainly interesting that they weren't done when they could have been, with the other EX-characters from their arcs...
(and did Goro just not fit on the Davy Back Fight EX-card?)