Hi Goran, this is a real personal thing because you know what sound you are getting from your series bass. From my personal experience of having series, signature and europa electronics in my basses I find the series basses to be the most rewarding of all to play because they have a particular sound that I cannot get from the other basses.
Option 1: Series I or II 5 string new or used
Option 2: If you are going for a non series 5 string, you will feel more comfortable with one that has two filters & q switches. Once you go down that route closest option to a series bass would be to find a 5 string with anniversary electronics which is the same as a series I layout but the electronics and pickups are different.
Option 3: Signature bass which you could modify to have anniversary electronics. ( I am currently in conversation with Mica to do that in the near future).
Option 4: Signature bass is a great option and extremely flexible tonally but different sounding to a series bass. ( I have the mod for 3 position q switch instead of two and that improved the tone for me)
Option 5: Europa/Rogue 5 - a great choice however, my own personal experience is that being used to two filter alembics I find the single filter easier to use live but less flexible tonally. (Not that anyone but me would notice)
If you are a 5 string player at heart and love the sound of your existing series bass, I think you will only be satisfied with a series 5 bass because you will hear the difference in the tones you can get between your existing series bass 4 and a non-series 5 string bass. I guarantee that if you buy a non-series 5 string now, in the future you WILL still want to buy a 5 string series bass.
Keep your eyes on here also :
https://www.station-musicshop.de/epages/64398533.sf/de_DE/?ObjectPath=/Shops/64398533/Categories/Category2/Electric_Bass/AlembicHappy hunting.