I might edit this topic later with screenshots. Also, I might add some more suggestion later.
(1) When you hover over a card in your hand it blocks the name of the card. The large image on the right side often goes unnoticed when looking at your hand on the left. It would be more convenient if the hover text was in a different place.
(2) The attack/defense of air creatures (in black) isn't always easy to read. A Chaos Wyrm with 8 power has the 8 stuck to his head.
It's also hard to notice antimatter on your air creatures aswell.
(3) In the right section of the main screen, if I hover over library, trade or redeem, it shows an explanation. When I hover over PvP it shows an unrelated tip. Is it random pvp if I don't enter the name of a player?
(4) Not so much a suggestion to UI, but the game keeps track of your pvp wins and losses. I'd like to not record my losses, allowing me to play casual decks and experiment without ruining my score. Another option is to have an unranked pvp option for people trying out new decks.
(5) Quests not completed are shown in red. A V checkmark for completed quests is easier to see and understand. If you completed a full quest line (the ice cave one for example) clicking that quest line again (the ice cave button) should tell you about your victory.
