While each commander typically exists to enable or otherwise set up a combo, Gretchen just wont see as many cards as Tatyova until youve already set up the infinite mana combo
for example, the Shrouded Fable sets booster were not released individually, and though you couldnt buy boosters as single packs, the cheapest way to get boosters was via mini tins, with two Shrouded Fable packs in each tin
So, for example KotS, a Dark to make Mudragon (then Bahamuth-Toad) or a Destiny Hero
Dont put this into a cube unless its hungry for everything other than the ending
Even though there arent any more Masters sets on the immediate horizon, WotC has made it clear that they will continue to reprint expensive cards at every possible opportunity
While it may be tempting to play two monsters and then wait for your next turn to roll around so you can drop a massive Level 8 monster like Dark Magician of Chaos or Superconductor Tyranno , you might want to consider Xyz Summoning with those monsters instead