And it is...
Mario and Luigi RPG 4, Download version.
I have played the past 3 games, by the way.
Anyways, I've only just started, but a few features I believe is new to the series that caught my attention.
Challenges to do in battle, such as getting Excellent through your Jump attack or avoiding Monster attacks 10 times in a row. You get points for clearing each challenge and upon reaching certain numbers, you get bonus equipment.
Badge System (not new completely, if I recall correctly):
You gain certain effects depending on your badge combination (one badge is main effect, other is how strong?). Your meter feels up after each attack and once you feel it up, you can use the effect. It can store up to two full meters.
And most importantly, the ability to save ANYWHERE anytime.
Very useful, but it makes...
the save block so obsolete, I don't know why it still exists...
In battles during the Dream World, Dream Luigi enter Mario during battle, increasing his HP and power of attacks.
Also, in the Dream World, certain attacks allow you to use the movement pad to avoid/counter attacks.
Other screenshots (all from the beginning of the game, so not really spoilers, I feel):