IIRC the only thing that 'could' have used more fleshing out is how they found Leafa. Sinon just randomly fell out of the sky at one point in the original (again iirc).
The Strea stuff is pretty much the same as the original IM, it just happens earlier in the game in HF.
If you're really that curious, just watch a vid/look up a fan trans because the combat is pretty slow and bleh in the original IM. HF improved in every single aspect for that.
Still story is pretty fun, more so if you're a fan of the anime of course otherwise you'll probably enjoy the combat far more.
Also Hollow Area wise: Complete it whenever but I'd recommend doing at least a few of the Floor Clearing chapter until you're used to the combat. The final couple of bosses in the Hollow Area are pretty nice and challenging imo (at around level 117-118 anyway)
.
Also another tip for later on, there is 3 'secret' characters in total. The one for completing the Hollow Area and then completing the floor clearing chapter. The other 2 can be found in the Hollow Area. I forget when but I know I was nearly finished the Hollow Area when I did one of theirs events. (being vague
)