For a game like this, how much would the sequel really differ by?
Having the same characters is essentially making the original game null. I don't think story will be anything major. "Instead of fighting blue boss, you fight red boss now!"
Sengoku Musou 4 is a good example of how they handle 'new' games. The original came out in March 2014 for PS3 and Vita. It was then ported to the PS4 and released in September 2014.
A few months later, in December 2014, the game's assets were used to make Sengoku Musou Chronicle 3, which was released for the Vita and 3DS. The game play was slightly different (more strategy oriented, as you control/command up to four characters in real time) and the story was changed up.
Then in February of this year, they released Sengoku Musou 4-II for PS3, PS4, and Vita. 4-II only adds in one new character and recycles a ton of content. There are new levels and stories added, however, balancing, and character leveling/progression was overhauled. They also took a mode straight from Chronicle 3, Infinite Castle, and added it into the game.
So in less than one year, there were
three versions of one game and a port to another console. There's also rumblings of a PC port coming.
I would assume that Dragon Quest Heroes II will probably follow the Sengoku Musou 4-II route. You'll see plenty of re-used assets, but there will be a new story and they'll add in characters/game play. If you don't care about the story stuff, of which DQH has more than your typical musou, then I would just skip the original and wait for the sequel. The only bad musou sequel I've played in recent heroes was Fist Of The North Star: Ken's Rage 2. And that wasn't made by Omega Force.