Naturally, this is only with Vanilla in mind, I ain't demanding anything tying into other mod biomes offhand or whatever Omnir adds himself for progression, jus tossing ideas around. Similar to suggested crafting materials tossed around.
FOREST - The neutral biome, non-elemental and probably some of the more basic summons could be made with basic Esper crafting materials here.
Valefor - I'm championing for this one, dangit. Easy early-game summon, probably requires easy-to-get materials and thus as an Esper, may not have the punch of it's bros until it's gotten more magnificence over the game.
Signature Moves - Sonic Wave, Energy Ray, Energy Blast
Bahamut - Yeah, can't forget this guy, and putting him as a purity Esper seems a bit weird, but on the other hand, you may need to craft him with multiple Esper fragments and perhaps even Pillar fragments to fully acquire him. If he has an upgrade, it's no doubt just one with Luminite if he's placed this far into progression. Not to be confused with Bahamut NEO, SIN or ZERO, which probably fit into Post-Moonlord but I ain't focusing on those ideas yet.
Signature Moves - Mega Flare
SNOW/ICE - Frigid wasteland, perfect for the ice-themed Espers and also rather early-game still to allow for a slight upgrade over Forest in terms of power. Probably common wielders of Frostburn and Freeze debuffs.
Shiva - Early-to-mid game acquire, would probably require substantial amounts of Ice and Snow blocks, maybe even a Frozen Mirror to reflect her beauty?
Signature Moves - Heavenly Strike, Diamond Dust
DESERT - Low-tier fire and thunder Espers could do good here for an early starting point.
Cactaur - Cactus! Lots of Cactus! Also needles! Lots of needles!
Signature Moves - 1000 Needles, 10000 Needles
Titan - Should be fitting enough for an earth figure to perhaps make the desert his home in Terraria? If all else fails, he can be put into the Caverns as well.
Signature Moves - Gaia's Wrath/Earthen Fury/Terrestrial Rage
Ixion - While Ramuh's lightning comes from the heavens, Ixion's thunder can easily be formed by the strikes on the earth, thus allowing potentially the thunder horse to be summoned as an Esper before the proper flagship thunder summon.
Signature Moves - Aerospark, Thor's Hammer
JUNGLE - More befitting for very specific esper types, such as the Magus Sisters getting linked to bee and beetle crafting materials, or Carbuncle perhaps matching with Cloryphyl ore for his requirements.
Carbuncle - Acquired mid to late game, Carbuncle is a support summon, offering primarily buffs to the player and those teamed with the summoner. As stated above, Cloryphyl ore would be a great represent for it's greenish hues.
Signature Moves - Ruby Light, Diamond Light, Emerald Light, Pearl Light
Magus Sisters - A trio of insect sisters, I'd make the suggestion to include Beetle Shell acquired from Golem in this particular recipe. Why have one biggirl when you can have three for the price of a slot?
Signature Moves - Passado, Camisade, Razzia, Delta Attack
CORRUPTION/CRIMSON - Evil, dark Espers reside in this twisted land, but much like the Desert, the far darkest foes could yet reside further in the earth.
Odin - While not usually seen as malevolent- though FF9 does mention him as a dark deity, so there you go- Odin's place in the dark elements of the world could make him worth daring early into it's depths. Could easily work as either early game or lategame depending how his Zantetsuken was balanced. Perhaps requires the Night's Edge in it's crafting recipe?
Signature Moves - Zantetsuken, Gungir
MUSHROOM - Perhaps something wild and trippy? I can't think of anything off the start that works here, but I'll sure to fix this as I find out.
OCEAN - Obviously, water-based Espers would be trickled from crafting points here.
Leviathan - Definitely a hardmode ocean-based Esper. Souls of Flight to help him get around, and then a mix of ocean crafting materials. No doubt primarily Shark Fins, but maybe a Pirate Map to confirm it is indeed hardmode.
Signature Moves - Tidal Wave/Tsunami,
UNDERWORLD - The most depraved of devils, and the lords of all fire all hide in the Underworld...
Ifrit - Goes without saying, really. Demonic AND a lot of fire! Perhaps to keep in line with his other trinity members, give him Underworld crafting materials but don't put Hellstone into the crafting recipe so an early hell-diver can still get the Esper without being at endgame for pre-hard.
Signature Moves - Hellfire/Inferno, Meteor Strike
Phoenix - Endgame summon from the depths of hell, another support summon that grants the unique power of Auto-Life to those allied with the summoner. Living fire may help keep this phoenix's plumage. Perhaps also has cure abilities?
Signature Moves - Phoenix Flame/Rebirth Flame
Diablos - A daemonic entity that resides in the depths of the earth, this destructive summon's a potential ally idea for endgame tier summoners, maybe.
Signature Moves - Dark Messenger
HALLOW - Holy summons of great light and power can be found in these hallowed grounds!
Mist Dragon - Reborn from the Hallow, the Mist Dragon could easily be a blend of Hallow materials like Pixie Dust and Space materials like Clouds, in order to form a powerful early-Hardmode summon
Signature Moves - Radiant Breath
Knights of the Round - A lategame summon for the Hallowed biome, Knights of the Round could be a devastating boss killer if balanced out with a sizable crafting requirement behind it.
Signature Moves - Ultimate End
DUNGEON - Much like the Evil biomes, the Dungeon offers a good way of adding cursed Espers into the mix.
Alexander - Constructed out of parts of the Dungeon's brick itself, and various metals, Alexander is a great asset to a summoner as a bombardment summon, well-known for it's devastating longranged firepower despite it's rather stationary nature at the best of times. Perhaps this could be alleviated with a staff to reposition Alexander as well?
Signature Moves - Divine Judgement
Doomtrain/Phantom Train - Choo choo mofos, this train of destruction ain't got no breaks! Well, unless your name is Saban of course. Either for early hardmode or Post Plantera, Doomtrain could make an interesting summon.
Signature Moves - Runaway Train, Diabolic Whistle
Gilgamesh - Major fan of the character, but this would be a great place for him to show up. My idea for this is mix him up with both requiring Ectoplasm (so Post-Plantera) and then requiring at least four great swords, either from the Omnir pack itself or the four strongest Terraria swords up to this point. Like, Horseman's Blade, Terra Blade, True Excalibur (of course) and the blade that comes with the Frost Moon I forgot the name of...
Signature Moves - Excalibur, Excalipoor, Masamune, Zantetsuken, Ultimate Illusion
METEORITE/SPACE - Lords of thunder and star gods gather around the signs and sights of the heavens.
Garuda - Bird ladies of doom and destruction, this relatively new Esper could work great for something to craft with harpy feathers, maybe even Big Harpy Feathers and Souls of Flight if you wish for Hardmode.
Signature Moves - Aerial Blast
Ramuh - Hiding out in the sky, Ramuh's crafting would no doubt incorporate rainclouds in order to represent elements of thunderclouds for his crafting ingredients.
Signature Moves - Judgement Bolt
Ark - FFIX's transforming airship is here for some air assault. I expect this summon would be best as a hovering high in the air support weapon, constantly barraging attacks from far above the user of the Esper. Easily at the endgame tier, possibly another one that requires Pillar fragments?
Signature Moves - Eternal Darkness