This is something I've been thinking about adding and describing for a while now:
Vs. Mode!
In this mode, similar to Office Mode in SP, you can set up a free Audition or Festival against the CPU or online.
You can select a solo idol, a trio, or even a quintet to perform (however, like in iM@S2, some songs will not be selectable if you go with a quintet).
You can play as ZetaPro's original 17 idols to start off, but you can unlock more idols by completing events in Story Mode. For example, once you defeat Kuroi's unit (Chiyo, Hotaru, and Karen) or the freelance idols (Setsuna, Kotone, and Kaori), you can play as them right away in Vs. Mode but you will have to finish your current story in order to play as them in Story Mode. You can even unlock idols from 765Pro and 876Pro (by completing their Special Festivals) and combine idols from all three companies to create an all-star unit! So a trio like Haruka + Ai + Sakura is possible!
If you are playing with a unit, team chemistry becomes a bit of a factor, though not as much as it does in Story Mode. Idols who get along with each other well get a stat boost. Likewise, their stats will drop a bit if they're not too fond of each other. This is to add a bit of a challenge when using units rather than performing solo.
As usual, before you go out and perform, you select an outfit/accessories (badges are not allowed in Vs. Mode) and a song from any you have bought/unlocked so far.
Up to six (minimum of two) users can play an online Audition (since it plays like it did in iM@S1) but only two users can play an online Festival. Gameplay is the same as always, just with human opponents.
If you don't want to go online and just hone your skills or get comfortable with a song you want to use, you can decide to face randomly-determined CPU opponents in either Normal or Hyper difficulty.
If you wish, your scores can be posted on online ranking boards. Each list is sorted by song.
DLC can allow you to play as 573Pro's idols as well as perform their songs--Shinkai Shoujou (for Erisa), Miracle Paint (for Rika), Sweet Devil (for Yoshiko), and Nekomimi Switch (for Koneko).