Oops, I meant *A lot of your creatures have 3 or more power.

I also like the idea of having Jarad, Golgari Lich Lord and Yargle and Multani in the deck to go along with the Lord of Extinction. It gives the deck a backup win condition outside of commander damage to help close out the game through a clogged up board.

I think another card worth considering is Titanoth Rex. Draw a card and permanently give your commander trample for just 2 mana while putting an 11 power body into the graveyard.

A lot of your creatures have 3 or more toughness so Elemental Bond/Tribute to the World Tree would be effects ways to draw more cards. Greater Good would also be a strong choice.

If you end up adding more sac effects as you develop the deck then Ghouls' Night Out might be a card worth considering. Being able to get up to 4 creatures for 5 mana is a good deal, even if they don't stick around after attacking with them. Sac them off for value or just don't attack so that you can make use of their abilities. If you end up reanimating Sepulchral Primordial as the card from your graveyard then that would end up being up to 7 creatures total for just 5 mana.

A card that I would recommend for a deck like this is Trudge Garden since the tokens it produces trigger the lifegain from your commander or other Soul Warden type creatures, giving you another potential activation. You could very quickly build up a large board state over a few turns.

Lathiel, the Bounteous Dawn might also be worth considering. Since its ability triggers at the end of opponents' turns too, having Soul Warden and other similar cards on the board means that Lathiel will also grow your creatures in response to all the creatures your opponents play. Lathiel combines really well with Spike Feeder to gain you increasing amounts of life at the end of each person's turn. I don't know if you are purposefully avoiding using infinite combos though, so it should be noted that Spike Feeder combos with Heliod, Sun-Crowned to gain you infinite life.