Honestly, just playing through the first few levels of official mode, you can tell so much is missing - for whatever reason, there are mecha that Koei just didn't bother to create, so you feel like whole chunks of story are missing in a single battlefield.
Take the Jaburo stage with Amuro's Gundam in official mode. In this stage, Char infiltrates the Federation's Jaburo HQ with his Z'Gok and you have to stop him...so far, so good. It's a large area and it certainly looks and feels like it was taken from the anime. But then you get to the enemies, and you notice - Koei didn't bother to make Acguys, so you're just fighting against Zakus, Doms, Goufs, and Z'Goks!

What's more, the "big boss" of this area in the anime (and all the other UC games that feature the Jaburo area), the Zock, is completely absent!

The Gyan is also totally absent...

And so are most (all?) of the mobile armors featured in the series. I don't know if it's because they don't fight in with the style of combat of the Musou series, or if it's just because Bandai's planning to do some sort of expansion in the future...
I guess if you're not familiar with the series, you might not notice all these absences, but I'm noticing it and thinking "Man, they better add these to the sequel." The weird thing is that for some reason they included a couple of MSV designs (a Gouf variation, Johnny Ridden's custom Zaku, and the Zaku Tanks), yet didn't include stuff like the Zaku I or the Guntank, which were featured rather prominently in the original series.
I'm guessing we'll see Gundam Musou Legends within the next year and a half, if not a full-blown sequel. There's just too much stuff missing here.
