Maybe they could be creative enough to have Jace inadvertently cause the next big bad.

Such as, Jace in his jace-ee brilliance thinks he can control Slivers for some sort of jace-like good. But, in a gremlin-like fashion, the plan goes awry and the Slivers get out/loose/self control back. They then use the Omenpaths Jace showed them (for some Jace purpose) to spread across the verse.

But instead we'll probably just get Tezzerett zapping Bolas to reset him or something and Jace does X and Y and both X and Y are pointless and dumb.

Honestly, after reading @IlLupo643's comment, Jace should be the next big bad (and ultimately die permanently). But Wizards is neither that brave or creative.

Would Tainted Remedy be worth an inclusion?

Wilderness Reclamation: Doubles your mana. The more you can take advantage of Slimefoot's mana sink, the better.

Jade Mage: Back up mana sink that's also cheaper.

Marauding Blight-Priest: Aristocrats win through attrition. This guy speeds everything up.

Mycoloth: Makes a stupid number of saprolings if he can survive a turn.

Ohran Frostfang: Great card draw. At worst, great trade against opponents creatures.

Savage Thallid: Great way to protect Slimefoot and fungus support. Occasionally gives you a saproling as a bonus.

Verdant Force: Not as cheap as Tendershoot but still gives you x4 saprolings a turn.

Keen Sense: Insane card draw.

Massacre Girl: Board wipe that also kills indestructible.

Best of luck with your build. If you have any questions about my list, feel free to ask.

Poison-tip Archer needs to come to main board. It works on any creatures dieing.

Here's my list for a reference.