Dark Mode Needed (Among other things)
I know many are talking about the appearance of the client and I completely agree how bare and barren it looks. but the biggest gripe for me is the always-on Light Mode.
Even though PTCGO didn't have a Dark Mode exactly, the blue was still easier on the eyes than massive amounts of white. Especially for people playing on the PC, I got a headache after a couple of hours of playing even in a well lit environment. Having a Dark Mode toggle would be great for those that need it, and especially on phones that have AMOLED screens they'd really take advantage of the black and darker colours to save power.
Of course there's much more visual tweaks that should be addressed, making the cards bigger and more visible, filling up the space more on PC to make it look less like a doctor's room etc. Pokémon has some of the best card artworks in any TCG game, and the client should reflect that. Maybe even make simple but effective custom boards/playmats for us to purchase and earn.