Thanks a lot to you both. I checked the links you pasted and am curious how it is best to proceed, as I cannot tell you on the “how” but only the “what” in case of the UI that I imagine.
A few keypoints I drew out of the examples you showed:
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What I called timeslider is some sort of feature I think no UI offers so far (atleast opensource)?
so creating this would be maybe of general interest. If I look at the Foss.events page I see a map and then an “upcoming events” section where I have to scroll down (so I lose the map). For the purpose of this page it totally fits the need. But imagine the usecase of what I was proposing: People looking for “what is going on the weekend” (that is normally 2-3 days in advance or even more sponatnous). For this case the timeslider would be nice. Also, you have more the impression of “exploration” in case you have no idea what to do than just scrolling some list + you see events going on nearby to your location.
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what I meant with “filter” was supposed to indicate the type of event, i.e. political, music, art, DIY whatever (in the picture what you just see indicated with local/global, these were filters I imagined in another context). What exact filters you would choose for categorization needs to be discussed and maybe is a perfect usecase for intersection of work with non-techies. Technically, timeslider and this sort of filters is the same, but we have just different apperances in the UI, correct?
@d3v0 you mentioned gancio UI as snappier. Dont know what exactly you mean by “snappier”, but the UI of mobilizon seems more attractive for my casual taste 
I cannot tell about the other features of gancio, though posting events anonymously seems interstesting at first sight, all use cases I can imagine of this are dealt with in private chats anyways, so I think this would be no big deal.
-Furthermore I saw Mobilizon implementing a feature for a calendar: Mobilizon Ouvre-Boites : Actus et Événements - Mobilizon
That would be a nice option to have next to the map.
- As for your concern with the amount of events. The goal is to have all events as I said. Seems idealistic, but I know someone always talking about dreamwalking
If you have the timeslide and another filter active at every moment I dont think it will be to much displayed at a time.
An addtional feature would be something like an “interested” (not to say like!) button, from which you could generate a filter that reflects the general interst for most people (that could work as default filter in case you have to reduce the events displayed but do not want to bias by lets say “music” category). I just saw Mobilizon has the feature of “particpating” but thats to rigid imo. People were not using that in my facebook era and just hit “maybe” anyways. Technically, it is probably kind of the same.
Finally, as we have an upcoming event in about 2 weeks where I would like to announce the idea to a broader audience, it would be nice to know on which basis it would be intelligent to start. So we could actually start gathering events. The map extension could come at a later stage, ideally just adding to the software/service/platform (no idea whats the correct wording here) that will be in use then already.