Crew 1 Cosima's ability triggers only if Cosima is on the battlefield at the beginning of your upkeep, and you may exile Cosima only if it is still on the battlefield as the ability resolves
Either that, or just use it to punish other players while their current board is empty
Even better, Jegantha, the Wellspring isn't just a free card in hand but actually does some powerful things in Niv decks, most notably tapping to cast Niv-Mizzet, Reborn itself (while being a 5/5 creature is also a nice bonus)
Searches and fetches become useless with mills from Ashiok, Dream Render. Meanwhile, while Narset, Parter of Veils and Spirit of the Labyrinth locks card-draw limit for all parties
(A creature with shadow can block or be blocked by only creatures with shadow.) Legendary Land Land (CTRL to add secondary) Recursion Animate Dead Enchant creature card in a graveyard When this Aura enters, if it's on the battlefield, it loses "enchant creature card in a graveyard" and gains "enchant creature put onto the battlefield with this Aura." Return enchanted creature card to the battlefield under your control and attach this Aura to it
For Competitive EDH Merciless Rage with Anje Falkenrath at the helm feels like the most easily adapted to a wicked competitive deck