This is how I see it:
Heroes from 1, 2, 4, 5, 6, 8, and 11 can work as alts.
Even Madason (aka, #5). I know he wields a staff in the boxart, and the overworld map. But there are two ways they could go about it.
1) Have his staff tied to his back while he uses a sword.
2) Pull the Ashley/Lip Mii costume Card, and use the staff like a sword.
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For Luninary, I could see him have the following alts:
1) Default purple
2) Initial garb (looks like Link's default color)
3) Eight's clothes, as in the DLC for DQXI.
4) Some advanced armor from XI.
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As for ability fidelity, let's not forget Ness can use Paula's PSI.
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I'd be excited for any DQ character. So if it isn't Erdrick, but it's some other protagonist, I'd be ok.