Been following this deck for a bit. If we're moving into Aether Meltdown mainboard, it may be worthwhile to consider dropping in 4 Aether Hub and bring in a copy of a payoff card like Dynavolt Tower .

Yorion decks usually have a tonne of draw and filtering, in order not to get mana flooded. Cards like Opt and Censor

It also makes them more consistent.

Lack of consistency is an issue for this deck. It just can't always get the right pieces in hand, when it needs to.

Part of the issue is the amount of counter spells. You need more removal, to deal with cards that slip through, and get cast. Something like Murderous Rider and/or Heartless Act (although Hero's Downfall can be better than Murderous Rider with Dig Through Time ).

Fatal Push on its own can't deal with larger Creatures, or Planeswalkers.

I would swap the lone Swamp for Urborg, Tomb of Yawgmoth as it is strictly better, and helps with colour fixing. Although you might then need to add another basic Swamp, so that Fabled Passage can find it.

At the moment Fabled Passage is fairly useless as you often have lots of Islands, and you only have one Swamp that you can find in your deck.

In my play testing Engulf the Shore wasn't great. I had a creature with Haste, and two equipment on the field. I just played that creature again on my turn, reattached the equipment, and hit as normal.

I see that you removed Negate which I consider one of the best counter spells in Pioneer. What was bad about it, for you?

Mystical Dispute is a great card for control decks, isn't it? Lots of Pioneer decks with that in the Sideboard. It's deadly against The Pioneer Blues deck I run.

Have you played this deck competitively? I would be interested to read some match reports to see whether or not I've been playing it wrong.

Hope that helps.

I should also mention that I've been using Test of Talents lately, along with Negate and I very much like what it does.