With so many Dominion expansions now (13 including base set), it would be wonderful if Dominion Online allowed for rated formats where you only play with the latest expansion (or some other officially-chosen subset, like a "standard" rotation). Similar to what other popular card games have done after they have been around for a while and keep releasing new cards (e.g. Magic: The Gathering and Hearthstone).
Personally I'm finally starting to be familiar with all the cards in the Silver subscription, but I'd rather skip learning all of the rules quirks and strategy subtleties in Adventurers, Empires, and Nocturne, and just jump directly to Renaissance. The rules of Dominion are just getting more complicated with each new expansion. Playing in person encourages only buying or playing with a couple of expansions at once, which works very well, but playing online requires playing with every card ever printed (at least, if you want to play competitively and/or in the official tournaments).