This suggestion will likely go ignored but I'm going to put it out there for the 100th time with some of my own Ideas on how to improve it.
The first thing I want to hit on, are the issues with the current version of the Codex. just flipping through the codex right now, there are many duplicate pages of monster cards, when each page should always contain new monsters and even among these pages there are monster cards that can not be obtained due to the monsters that dropped them being removed.
I think It's Vital that the pages be re-managed and old cards (not familiars) be removed from the main part of the codex (will expand on this idea later). Doing these things will make it much easier to view what cards still need collecting for sets. there also needs to be some kind of internal developer system or Reminder that when an area is getting removed or completely revamped (See Kerning Square, Mushroom Castle..etc) that any non-returning monster cards should be removed from the card collection count, pages of the collection codex and Set Collections; However, I think that there should also be a "Retro Codex Collections" page dedicated to the removed codex cards, where, It's only purpose is for collectors who like to collect them. If the "Retro Collections" be comes a feature it would also make sense to have a "Retro Cards" pack that you can buy for meso from some vender.
The second issue I would like to hit on , are missing cards, familiars, sets, and drop data for the "new" current mobs in the game.
I understand that it might be hard to get things like this implemented right when the changes happen but it clearly can happen (looking at those Dark World Tree familiars) so I believe the next step to improving the codex is to at least roll out the monster cards of mobs not in the codex, as well as, collection sets for the areas they are located in so we can also have drop data for these mobs. The familiars should come after but I can understand a delay with them due to new abilitys needing to be added to each one; However, I know that other versions (CMS, TMS and JMS) already have this covered for a lot of content already so it would be more effective to take what they have for card data, set data, and familiar data and translate it.
The third thing I want to hit on are some core features of the Codex and Familiars that are in other versions.
What many of us players have noticed and also known about for a long time are the very distinct features of some familiars that only work if you can summon more than a single familiar, these abilitys come from JMS where they can summon more than 1 familiar at time among other things; however, the drawback is that you have to "fuse" the monster cards together before you add them to your collection. I think it would be great if GMS had this feature, but allowed us to summon any 3 of any familiar already in our collection.
I will point out that JMS did also have a familiar potential system added to their familiars in the same update, where potential was added to any familiar card that dropped or came from a pack, and low cost familiar cubes (only like 100~300 points iirc in JMS) but I dont think that's necessary for us (but I would rather have the system be on the same footing as JMS so it can still be maintained and prevent the issues we currently have)
There are also the CMS and TMS versions of the codex, that have a different UI and some new systems. While they dont allow for summoning 3 familiars they do allow you to have 3 codex set effects on at once. Which is a neat feature but I dont agree with the other features of the CMS and TMS version which were centered around familiar potential, but the UI they have is good and clean.