ESO is one of the biggest titles in the MMO scene. The fantasy lore is medieval in its core, with magic, elves, orcs, dragons and knights. And always has been. Freeroaming and the option to play what you want and how you want in this world while staying close to a lot of other similar fantasy universes (f.e. D&D) makes a lot of fantasy fans happy, since you don't have anything close to what ESO already does in one package. Which is a good thing. You can do solo questing in overland PvE. Many different stories big or small are to be told there. You can make friends via guilds or group finder and do some dungeon diving or harder content with even more other players by going through trials. You have the option solo queue for small scale PvP aka battlegrounds. You got a semi PvP/PvE mixture called imperial city. Or the biggest PvP option with large scale battles of 3 factions which is Cyrodiil. You can mix and match classes, pick between different playstyles wether its sword and board, bow, a caster or a healer. Combine the sets you want to get closer to master what you aiming for. Make it effective via finding some of the best gear through the minigames (like Antiquities) or upgrade your things via crafting. Could also just lean back without fighting and play the auction house game (or in this case, guild trader game). Or just buy a house from the crown store, build or buy the furniture and make a place you can call home for your ingame character. And there is so much more that I didn't list.
With all these things in mind, the game made it through the last 8 years. With ups and downs. To the point where we are at now. Everything sounds cool on paper. It sounds cool if you tell your friends about every details you can do. Until you notice that its already so heavily packed with stuff to do, that with every new content that came with DLC's and updates the game got more and more bugged, unreliable, and chaotic. Two of the most important details are what leads to the current hate as well as to a more or less unhappy feeling about what coming with the High Isle:
First comes the server stability causing huge impact on the gameplay experience itself. Many content of the game gets close to unplayable without this getting fixed. All the cool builds and playstyles have to adjusted to what the server actually allows you to do.
Next comes the balancing. There is more than just your race and class playing a role with how effective you can be in either PvE or PvP. The sets you pick, the glyphs you put on them, the traits, the skills you chose, the CP you got and the CP talents you equip, the potions you drink and the buff food you eat. General survivability, damage capability, mobility or flexibility. All these things come together to the point where people run what the community knows under the term Meta (Most effective tactic available). Since this game is so heavily packed with different options, you could get the impression that something like a Meta sounds pretty restricting. And it is. Because if you are not part of the Meta, you risk not being accepted by the community itself to do all the nice content that you have available. So in short; you have to do the same if you want certain achievements, you have to do the same if you want to be on the winning side of PvP, you even have to adjust depending on the different situations to get the harder PvE content done. All in all you can also just leave behind what you think is not your cup of tea. But in the end you are forced to certain builds to not somehow mess things up. All comes down to the Meta again.
The thing with the Meta though is, its always shifting. New sets mean, new Meta. A change of the combat system for whatever reason means, new Meta. Animation cancelling, light attack weaving, line of sight. Just to name a few details that play a role there. The root problem of some of these points can be found when looking at the unreliable stability.
So, what does the community really want now?
To sum it up: Server stability, bugs getting fixed (in time), balancing, new classes, new weapons, and truely organzied group play in an epic PvE and PvP adventure paired with different playstyle options and action that lets people shiver in their favorite game makes us want more and keeps your players playing and paying.
Going through in detail, the absolute most important thing to everyone (and I think I can speak for everyone in this case) is three words: A working product.
Without having a fluently functioning game, the whole gaming experience will lead to a tough time. It should be a fun time, though. Regardless of what you are interested in doing within the game, right? Well, then the servers have to get taken care of once and for all. Its bottlenecking everyones gaming experience! Moreso in PvP than in PvE. Even though we could see many trial guilds as a whole needed to completely cancel their plans on running your content. And in terms of PvP, it leads to players leaving Cyrodiil out of frustration once they see ball groups stacking heals and buffs and causing lag for others on purpose to bomb them down soon after. Call it exploiting, call it tactic. I call it a mess. And we all either gotta adjust to these things once more, or (the more likely option) people just quit and play something else that is at least working reliably in its core content.
We need a functioning and fluent PvE experience. We need a functioning and fluent PvP experience. Classes got promised. Weapons got promised. PvP content got promised. Performance got promised. Balancing got promised. Where is it? That's what players were asking for. Its nowhere to be seen. Why? No answer on these. The community feels ignored at the most important topics. Now a card game is part of the newest DLC main feature instead. Card games are of a niche sort anyways. Nobody asked for it. But now we get it. More content is a good thing in general. But the absolute majority of ESO players didn't even finish half of what the game already has to offer right now! Its so packed with things to do, you need days to read through all the guides and use addons to even understand about all the possibilites we have.
PvE is important. Trials are a fun addition for larger scaled groups. The organization part can messy sometimes though. Spare your players some of the hassle to organize it and implement a well thought out trial finder (players fear fake tanks and healers there though, so might as well adjust the system to recognize true tank and healer specs with it). But PvP is just as well. Never underestimate the potential of your PvP again. One of the biggest PvP titles out there in the MMO scene is a proof in itself that money can be successfully made with PvP alone, and that is Albion. Millions of players worldwide. Main focus is indeed PvP. Do you want your community to play games like this instead? I highly doubt it. So why not polish the PvP system as a whole? A rework. An overhauled Cyrodiil. Overhauled battlegrounds. You could even go for a light version of this. High Isle and its knightly Orders would be the perfect atmopshere to implement things like new kinds of faction wars. Dark Knights vs Paladins for example. Noble vs lower class. Intrigue systems with players getting involved. A return of the thieves guild and dark brotherhood in a medieval world. A minigame that is related to fighting with the characters a lot of players spent a lot of time, effort and money into finetuning would have gotten accepted way better and would have fit much nicer into the medieval Breton-Knight theme than dueling someone with cards. You already stated medieval history are your inspiration. It was a dark age. But somehow you got the idea to plant sunflowers all over the place? Doesn't seem very intimidating if you ask me. Druids fit there indeed. Volcanos though? It seems out of place. Graveyards with undead constantly raising from its rest and attacking and threatining their citizens would be. A truely dark atmosphere (The Dark Souls series is not successful for its difficulty alone, but rather for its dark knightish medieval and dangerous atmosphere. High Isle could fill this hole for the MMO RPG fans out there since the devs of this series didn't yet offer that kind of thing yet).
If you ask me, if I can at least understand the basics of what the community wants at an absolute minimum and can also bring up ideas to polish what is already in the game, then I am sure qualified workers can do even better. ESO has the potential to grow again to an absolute epic level of fun. All you gotta do is listen to your community, implement what the community asked for after all these years, and when things got polished then additions like this card game would find at least a somewhat better acceptance within your community too.