When I was younger I had 3 Yu-Gi-Oh games on DS - This one, Spirit Caller and World Championship 2007. For years I've had the opinion that Spirit Caller was my favourite. After replaying both that and this, I can say my memory did this game dirty.
Duel World mode is exactly what I want in a Yu-Gi-Oh game. It isn't really a story mode, it's sort of just maps with duel monsters that you can duel, who use a variety of decks generally focused on themselves as their key card. But what makes it great is that there's a ton of little gimmicks in each world. For example in the very first world there's a colosseum where you use structure decks (based on the ones from the real world at the time) to duel the AI with another random structure deck. It's a great way to earn the in-universe currency early on when your own deck would be lacking since you have 13 ready-made decks to use. And if you win with all 13 you get an extra reward, which is a single copy of every Amazonness card! Though admittedly that reward sets a standard not really met by any other challenge in the game. You can get cards from actions in the duel world, but it's generally just a single copy of a random card and never a full playset of an archetype. For example in one world there's a monster who is being "burned" by some fire-related card, and if you win the duel you can save him and get that card. There are chests in some maps which could contain a card (usually chest-related), fight a mimic monster, have DP (the currency of the game) or nothing. Even just winning against random duellists have a change of giving you a card related to them/their deck.
Other fun challenges in these worlds include things like duel puzzles, consecutive duels where life points carry over, having to use a monster-only deck, in one world every duel is a tag duel, and many more.
Not everything is a gimmick duel though (except tag duels in the 4th world). There's plenty of spirits around to have regular duels against, but these additional challenges are just a fun way to shake things up and get you to try out different decks.
As I said though the rewards are never as good as the colosseum makes you think they'll be. Generally they'll give you an anime character to duel in "World Championship" mode. The mode the game is named after. IN that mode you can...just kinda duel opponents and nothing else. It does eventually unlock 8-bracket tournaments, but otherwise there's no real difference from dueling here to dueling in duel world. Of course the opponents here are different (beating a spirit in DW mode 5 times unlocks an "upgraded" version of them in WC mode, so like beating Skull Servent 5 times with unlock King of Skull Servants. And as mentioned there's anime characters here who aren't in Duel World).
Depending on how much completion rating you feel like doing the game is actually super non-grindy. I think you only ever need to beat every opponent once each to get to the end. Of course unlocking more stuff will have a bit of a grind. As mentioned unlocking opponents in World Championship mode will involve beating everyone in Duel World 5 times each. Beating X opponents in WC mode unlocks packs too. You DO unlock packs every time you reach a new world so it's not like doing the bare minimum would leave you stranded with your starter deck, so it's really just rewarding the grind with more options.
Actually getting DP in this game is a little different than Spirit Caller. In the latter, along with all regular bonuses post-duel you got a big bonus every time you levelled up where you'd get DP = to your new level x 100. In this game there is no levels, so instead you get DP equal to the amount of total duels you've had divided by 10. It starts very slow but eventually you'll start getting a lot of money for every duel. However there's also a consecutive win bonus. Let me say this right now - if you want to cheese the system just save after every battle and do a reset to the menu whenever you lose a duel. You can effectively go the whole game with a full consecutive win bonus which will result in massive amounts of cash the further you get. I was too dumb to do that, but if you want to cheat your way through a 16 year old game in a very niche franchise, there's your way.
Anyway that's all I have to say. Duel World mode is fun and much better than the dragged out story modes of Nightmare Troubadour and Spirit Caller.
Duel World mode is exactly what I want in a Yu-Gi-Oh game. It isn't really a story mode, it's sort of just maps with duel monsters that you can duel, who use a variety of decks generally focused on themselves as their key card. But what makes it great is that there's a ton of little gimmicks in each world. For example in the very first world there's a colosseum where you use structure decks (based on the ones from the real world at the time) to duel the AI with another random structure deck. It's a great way to earn the in-universe currency early on when your own deck would be lacking since you have 13 ready-made decks to use. And if you win with all 13 you get an extra reward, which is a single copy of every Amazonness card! Though admittedly that reward sets a standard not really met by any other challenge in the game. You can get cards from actions in the duel world, but it's generally just a single copy of a random card and never a full playset of an archetype. For example in one world there's a monster who is being "burned" by some fire-related card, and if you win the duel you can save him and get that card. There are chests in some maps which could contain a card (usually chest-related), fight a mimic monster, have DP (the currency of the game) or nothing. Even just winning against random duellists have a change of giving you a card related to them/their deck.
Other fun challenges in these worlds include things like duel puzzles, consecutive duels where life points carry over, having to use a monster-only deck, in one world every duel is a tag duel, and many more.
Not everything is a gimmick duel though (except tag duels in the 4th world). There's plenty of spirits around to have regular duels against, but these additional challenges are just a fun way to shake things up and get you to try out different decks.
As I said though the rewards are never as good as the colosseum makes you think they'll be. Generally they'll give you an anime character to duel in "World Championship" mode. The mode the game is named after. IN that mode you can...just kinda duel opponents and nothing else. It does eventually unlock 8-bracket tournaments, but otherwise there's no real difference from dueling here to dueling in duel world. Of course the opponents here are different (beating a spirit in DW mode 5 times unlocks an "upgraded" version of them in WC mode, so like beating Skull Servent 5 times with unlock King of Skull Servants. And as mentioned there's anime characters here who aren't in Duel World).
Depending on how much completion rating you feel like doing the game is actually super non-grindy. I think you only ever need to beat every opponent once each to get to the end. Of course unlocking more stuff will have a bit of a grind. As mentioned unlocking opponents in World Championship mode will involve beating everyone in Duel World 5 times each. Beating X opponents in WC mode unlocks packs too. You DO unlock packs every time you reach a new world so it's not like doing the bare minimum would leave you stranded with your starter deck, so it's really just rewarding the grind with more options.
Actually getting DP in this game is a little different than Spirit Caller. In the latter, along with all regular bonuses post-duel you got a big bonus every time you levelled up where you'd get DP = to your new level x 100. In this game there is no levels, so instead you get DP equal to the amount of total duels you've had divided by 10. It starts very slow but eventually you'll start getting a lot of money for every duel. However there's also a consecutive win bonus. Let me say this right now - if you want to cheese the system just save after every battle and do a reset to the menu whenever you lose a duel. You can effectively go the whole game with a full consecutive win bonus which will result in massive amounts of cash the further you get. I was too dumb to do that, but if you want to cheat your way through a 16 year old game in a very niche franchise, there's your way.
Anyway that's all I have to say. Duel World mode is fun and much better than the dragged out story modes of Nightmare Troubadour and Spirit Caller.