The fact that the Arcana vote even needs "construction" would suggest to me that it won't simply be a list of every hero. Probably at least a curated selection they have ideas for. Maybe even a hint or preview of what the item itself might be?
I don't think there will be a full list of heroes, but the furthest I see it going is some concept art to choose between. Valve are clever enough to know that allowing for any hero to be picked could alter the perception of that hero and affect how much it's played.