Obviously it can't be the end.
Tons of other anime, TV shows, and some games pull the cliffhanger thing. The rest is for the viewer or player to think up for themselves. Also, while Episode 6 may be the end of the main storyline, it wont stop them from making more content related to the main plot. Like -ahem- OVAs (most of which I despise but oh well LOL) and etc.
Can't wait to struggle my way through all those bosses, as a Soul Master I'm sure I am going to have to (if I play fair /no skunk with guardian). Though, I have a feeling unless these bosses in Episode 6 have a LOOOOOOOT of resistance, a ton of players are just going to take them out in 1 or 2 hits.
I really want to see the Sacred Esper too...