I will be disappointed if they do not bring back trap cards, such as
Archive Trap. Anyone who has ever played D&D knows that you have to be on the lookout for traps! And on a similar tangent, I look to see at least one
Flash based rogue that "traps" when a trap is activated to
cancel the effect.
I agree on Level Up needing to come back, full on flavorful.
In terms of creatures, they had better have some killer dragons. No it's, ands, or buts about it. Dragons must shine. But I would also like to see Mindflayers focused on Mill strategies. Maybe they grow when a library is milled (particularly your own), or maybe they "eat" (ie,
exile) milled cards. Maybe they can take over creatures with toughness lower than the number of cards milled at the end of the turn. So many options...
Lore wise, I would like to see Sorin arrive, perhaps introducing Sigarda as a newly awakened Planeswalker, only to be chased by Nahiri hiring heroes to try to kill him (he would be a vampire in a D&D world). Maybe see Vraska, since we aren't really sure where she went to...