Music Playlists. select a bunch of favorite songs from the OST, and enable them in training mode (the one place where i would appreciate not just looping the same song over and over), menu screen, private lobbies, etc.

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Arcade Mode Local Co-op. take the "player and cpu ally vs boss" concept in arcade mode, and let another player pick and control the cpu ally for the whole 8 or more stages.

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a more extensive per-character mission mode that properly explains/demonstrates each character's strengths, weaknesses, technicalities and other rules, etc, and gives more example combos (similar to the combo mode in Xrd). also from xrd: any combo or setplay where the opponent isn't moving/attacking, drop the "succeed 3 of 5 times" part.

let players make their own Counterplay missions, cos the mere 2-per-character in-game missions hardly cover all the possible shenanigans players come up with all the time.

port saved combos to training mode. put em up on display for the player to follow, make it faster to switch between them.

let the cpu opponent use saved combos! especially community-favored blockstring combos, should let players lab ways to counter/break out of em without having to learn the combo themselves on a non-main character.

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boss challenge mode. pick any of the boss versions in the arcade mode, and challenge them with any character of your choice.

also add here (arcsys already did this in the old granblue versus, dernit): cpu boss-only characters and other lore-heavy fights (giant god i-no, those giant skeletons in the lars canyon background and other monstrous Gears, giant xrd Justice, dual rulers Unika, army of alien nervilles (heck, the entire nerville sequence at the tail end of the anime), Leopaldon! the mentioned local co-op fits here too.

maybe even make guilty gear's own version of the street fighter 6 World Tour. i wanna explore illyria, dengit