Well eventho I would've loved Isabel to turn out the sort of dark-evil, but good character, I can't say I am really disappointed now as the plot has thousands of ways to fo, and most aren't bad at all. Depends on how the producers will play their cards.
I would've loved to see her being dark tho, but not necessarily evil or good for that matter. Unfo that combo is pretty rare to non-existent on tv or else where much. It would've broken the stereotype or good and evil characters a little. But that's a bit offtopic.
Anyways, I wonder indeed what will become of Isabel's character and whether Malcolm will fight Deathstroke at the finale.
I know it sounds crazy, but wouldn't it be really cool and unpredictable if he teams with team Arrow, hence they seem unable to match team Deathstroke by any means, not even getting close. So unless they get some outer help, they appear to be terminal.
As for Isabel, I doubt they will put her on top of Queen Consolidated just to cut her head off at the end of the season. Given that Summer's play skills are awesome and really adds up to this character and don't forget that Moira has yet another secret in her pocket this time concerning Isabel.
If they go and cut her from the script that line of Moira would look like a complete bs and won't make up anything positive or sensible for that matter. So I am sure that unless those things unveils till the end of this season, we will see Summer in the next season as well. The fact that "we’re going to start Season 3 with Oliver in very different circumstances than he’s been before, which, obviously, him being in different circumstances changes the circumstances of his paid bodyguard [Diggle]and his paid assistant [Felicity]since he can no longer pay them", meaning he being broke, then that's a near guarantee.
It would be interesting if the collision between Slade and Oliver doesn't "fully" unfold at the end of this season for some force-major reason, don't you think. Like something completely unexpected crosses their path to one another, to such extend they might even have to pause their feud and unite for the time being. But I doubt that this fits what the start of season 3 is promised to be somehow. But who knows.