The sideboard is where the deck needs the most help, since I’ve never played the format before. I know it’s almost impossible to make a good sideboard without knowing what your local meta looks like.
I just went to mtggoldfish and looked at all the top meta decks and constructed a sideboard to help against those decks. Kinda tried to cover all the bases until I can make a more local meta oriented board.
Tormod's Crypt
is good against Phoenix and
Kroxa, Titan of Death's Hunger
. It can also be brought in against Lurrus decks but probably not worth it there. I’m unsure if this spot should be
Soul-Guide Lantern
or crypt. Crypt is a free prowess trigger but Lantern is better overall.
Alpine Moon
is for the
Lotus Field
deck. Don’t know anything else I would bring it in against.
Ceremonious Rejection
is mainly for the
Karn, the Great Creator
decks. It can counter Karn or most of the things he grabs from the board.
Fry
is to deal with
Narset, Parter of Veils
. I’m not sure which other Planeswalkers are played in Pioneer. Not sure what else it hits but could come in handy.
Magma Spray
is just another burn spell for small creatures. Like mana dorks and such. It really shines against the recur-able threats in mono-black aggro like
Bloodsoaked Champion
,
Scrapheap Scrounger
, and
Dread Wanderer
. It can also help against Lurrus decks.
Lava Coil
can take care of some bigger threats the deck normally has problems with like
Steel Leaf Champion
and
Omnath, Locus of Creation
.
Spell Pierce
is just a catch all, good against counter spells, Planeswalkers, and removal spells.
Aether Gust
is good against 5-Color Niv, Kroxa, mono-green, and
Lovestruck Beast
. Among other things.
Fevered Visions
I copied from claudiohmtg. It’s good against control decks. It’s a non-creature threat that doesn’t die to sweepers. It’s also my favorite card of all time.