For me, one of the most off putting parts of oETG at first was the tab layout. It was confusing and didn't make a lot of sense at first. Of course, now that im used to it it's perfectly fine, but it is pretty confusing for beginners(or, it was for me at least).
(A lot of the things I am going to suggest here are going to be very nitpicky and might seem really un-needed, but small changes in layout can make a huge difference when you are first starting out in a game, and my goal here is to try and give feedback on how to make these small changes from the perspective of a newer player. I have been part of a few other game design and balancing projects, so I do have SOME experience with this sort of thing. I love this game, and I may appear to be kind of judgmental with some of these nitpicky things, but I promise that I am only trying to give as much constructive criticism as possible without leaving anything out. If you disagree, then that's fine, layout design is partially subjective so it's natural to have disagreements.)
The first thing I suggest doing is not opening up to sandbox mode, I didn't even realize that there was a game to it besides the trainer, and I'm sure a lot of people have felt the same way to first. So, instead of opening to up to trainer, open up to a login/registration screen, then once registered, have a tab to the side that says "Trainer/Sandbox mode." that would turn the game into the current opening screen until you logged back in.
The second thing I suggest is separating the Colosseum and quests tab from the "Regular" AI battles tab. Put them side by side in similar blue boxes, except have the title "AI battle" above them encompassing them both within the same category, but still making them differentiated. The point of this is allowing people to differentiate between the regular grinding AI's from the quests part of the game, since that is likely the first thing people will be looking for when they start playing(since they are probably used to the original game). I will try giving a visual for this to give a better understanding of this later.
Next, I suggest moving the wealth T50 tab somewhere away for the shop, editor, and upgrades tab(I suggest putting it in the right hand corner of the options box since there is open space). It's not as important to the game as those 3 things so it doesn't really belong there and is kind of distracting. Additionally, change the "Editor" tab to say "Deck Editor." If that's too similar to ETG, then maybe something like "Manage Deck" or something along those lines, just to make it stand out more then "Editor." The purpose of this change is to draw the players attention to the word "Deck" since that is the first word they are looking for when they want to change their decks.
As for the arena box, there are 3 pairs of tabs with the same name, that's kind of awkward. Simply adding the number "2" to the end of every tab on the second row is probably enough to fix that.
Finally, the pvp box. The only really thing that could improve this is adding "/Trade" in the challenge box so it says "Challenge/Trade."
That's about everything I have for now. The last thing the game needs is a tutorial and some text for newer players, which I am hoping to start working on and have finished by the end of today or tomorrow. Thoughts and feedback are encouraged, thanks for reading. <3