Looking at midrange all-stars like Den Protector and Sylvan Advocate, its easy to see what makes a good midrange card: efficiency or guaranteed card advantage
And at the beginning of each end step, if any creature card was put into your graveyard that turn, he creates a 1/1 Devil token that deals 1 damage to any target when it dies
Smee card could be quite useful, helping you gain incremental lore without any adverse effects (assuming you manage to keep another pirate character in play at all times)
Creature - Dragon Burn (CTRL to add secondary) Creature Klauth, Unrivaled Ancient Flying, haste Whenever Klauth attacks, add X mana in any combination of colors, where X is the total power of attacking creatures
Lurrus shines in low-to-the-ground decks with lots of 1- and 2-mana cards, usually aura decks in Historic
I added a dedicated step using MTGLandGenerator : It generates a specified number ( land_variations_per_type ) of variations for each basic land type