Increase the number of saved decks from 100
Arena boasts all sorts of game formats. Standard, Historic, Alchemy, Brawl, Limited, Explorer not counting any Midweek Magic events. Almost all of these can be played in Best of One, or Best of Three with very different ways to build the decks depending on that method. Most people, especially whenever a new set comes out or someone feels like making a deck brew can have more than 10-20 decks for each of these formats alone!
It's no surprise when you add that all up why 100 saved deck lots is too small and discourages experimentation when you need to export your deck to another site just to continue building. Even increasing it to 150 would be plenty, but 200 would be ideal.