Would it be possible to develope so you can configure filter-options directly in a sensor?
That way one could reduce the amount om entities in hass, wich already have a tendency to be, hm, many.
What do you mean, exactly?
How do you see it?
Now you have to create a sensor for the filtered value, so you will have two entities for one thing.
I would like the option to somehow set a filter on the original sensor without having to create yet another entity.
I do see the problem with sensors of so many differen types, manufacturers etc, but I feel my Hass is crammed with entities, but one could always ask 
But what do you mean exactly by “filtered values”? Filtered on what criterias?
Provide examples so that anyone that could implement your FR has a clearer view on your requirements.
On a side note, I don’ really see the link with the ever expanding number of entities (I assume you know you can disable the ones you’re not interested in).
Ah, sorry, easy to forget that other poeple can’t read your mind 
I mean this type of filter Filter, that will filter out spikes. One spike in for example power meter will make your data rater usless, and for quite some time as well.
I have several z-wave power plugs, and some of them are more prone to sending spikes than others, and since you can’t delete data, at least not to my knowledge, it will make for example the energy dashboard not that useful when your fridge has consumed 1 247 896 kWh…
And can you disable the sensor that you apply the filter to?
Clearer now 
No. A disabled entity basically disappear.