After some considerable back-and-forth between several options, and some input from the MTGS Community, I was eventually able to settle on Jalira as my mono-blue commander.
The beauty of Jalira is that her ability does not see Legendary creatures, so I was able to still run a lot of other Legends that I was debating in the maindeck without diluting the pool of cards for Jalira to polymorph into.
There were a couple of under-performers in the deck that I was looking to upgrade with some cool new toys from Amonket and Ixalan block.
Amonkhet As Foretold - I expected to try this and not like it, but it was honestly pretty good. In a deck full of Instants, being able to counter something or draw some cards while tapped out on my opponent's turn was quite nice, and this deck makes the game last long enough where it can ramp up time counters to cast some of the more expensive cards in the deck. Commit//Memory - A very useful spell to help protect my position in the mid-game or to reload my hand (and library) in the late-game. Oketra's Monument - I obviously have no use for the first ability, but the second ability of creating free 1/1 tokens was perfect. The issue I was running into is that the deck doesn't actually cast a lot of creatures, so this doesn't trigger enough to be worth it.
Hour of Devastation Scavenger Grounds - Having access to some graveyard hate is a nice addition. Swarm Intelligence - I tried this and whenever it came it it was too little, too late. That 7 mana cost keeps it from being broken, but it also keeps it from being really useful.
Ixalan Chart a Course - I like this card as another cheap way to dig to business spells. Primal Amulet - When I started using this, I had 28 Instants/Sorceries in the deck and only 9 of them were things that I really wouldn't want or need to copy. I made sure to up the count a little bit and this earned a spot in the deck. River's Rebuke - I tried this in place of a few of the other mass-bounce spells and I found it was pretty good (especially just before swinging with an Eldrazi).
Rivals of Ixalan Arch of Orazca - The deck had some room to run another colorless land, so I stuck this one in there. Golden Guardian/Gold-Forge Garrison - I like the idea of the flipside on this, but getting it to flip in this deck was just impossible.
There were some good cards to try out from Dominaria and some cards that I want to try from Battlebond:
Dominaria Karn, Scion of Urza - This is a much better token sacrificial fodder creator than a lot of other things I'm running, plus it has the ability to create huge monsters itself with the various artifacts that the deck runs and/or creates. Coupled with the fact that it is also a card draw engine and it leads me to believe that this is a card that every Jalira deck should be running.
Karn's Temporal Sundering - I think that I run barely enough Legendary creatures and Planeswalkers to make this playable, but only if Karn and Naru Meha make the deck (pushing the count to 13 maindeck plus Jalira).
Naru Meha, Master Wizard - There are plenty of spells I want to copy, plenty of Wizards that will get the bonus, and it is Legendary so it doesn't affect Jalira, so it made sense to give this a whirl.
Blackblade Reforged - Not that the bigger non-Legendary monsters need to actually get bigger, but its worth noting that a deck in my configuration has plenty of the Legendary cards that can equip this on the cheap can use the boost.
Memorial to Genius - I'm not a huge fan of the Memorial lands, but his deck runs one one ETB tapped land and has plenty of space for something like this.
Battlebond Arcane Artisan - I think this card is potentially fantastic at producing potentially huge creatures on the cheap, especially given that one of the weaknesses of the deck is when a big monster gets stuck in hand. I just have to decide whether this is worth diluting the pool of cards for Jalira.
Spellseeker - I really like the effect, but I'm adamant that I don't want to run any non-Legendary creatures that I would mind getting when I activate Jalira, so I'm unlikely going to want to play this.
Zndrsplt's Judgment - This card seems fantastic and I really want to try it out here. I'm worried that the number of Legendary creatures is too high (a lot less value copying a Legendary creature), but the threat of copying an Eldrazi and having everyone else bounce something seems really good.