October (Ep5) is when weapons that are realistically obtainable and better than Nemesis/Slave will start to be released.
There will be a few weapons with long term upgrade paths, for example, the first Ep5 urgent boss will drop a series of 13* weapons that are upgradeable into 14*, and as the game reaches the end of Ep5, those weapons will be further upgradeable into 15*.
Nemesis and Slave also have their own upgrade paths into 15* (together with 2 other weapons, a total of 4 weapons will be needed for that upgrade).
Then there's the ep5 final boss, which will also have a 13* series of weapons that are upgradeable into 14* and 15*.
Note that both the weapons from the first Ep5 boss and the ones from the final Ep5 boss will require materials that are either extremely rare or distributed through limited methods (like, obtaining those mats from title rewards, event campaigns and such).