Umm, there are many possibilities, however for any 1520 owner like myself who's gotten this phone at its launch, and paid premium, there has to be a massive improvement in every aspect to attract me to a newer replacement, and as far as I can see Microsoft have this poker face strategy with the future of Windows phone's upcoming flagship phones and even the Windows 10 for phones (ridiculous naming I might add) which says either they have a very well drawn plan and this is a strategy of say less (or nothing) and do more (not much doing in the higher end for sometime though) OR they have no clue on what to do, and tracing their older steps and Nokia's, I'm not expecting a device that can replace the 1520, due to the their whole "not using the latest hardware in the market for their phones, but still our phones are fast" which is weird but seems to be working.
Anyway I'd rather be here than be on or with the Apple or Android wagon, and I still have faith in the Windows phone future and Microsoft's strategies.