>What ist this futurehit-function for?

It is for properly reproducing instruments which give off substantial sound *before* the stroke (which is in sync with the music).
Without futurehit you would have two possible approaches:
1- Cut off the initial part of a ‘shake’ so that the MIDI hit always results in a sound in sync with the music.
The downside is that it sounds a bit like playing a maracas by hitting it with a plastic mallet instead of shaking it.
2- Keep the initial build up of a shake in the sounds played but position your MIDI notes earlier, out of sync with the music, to compensate.
The downside is that if you change the tempo of your song the MIDI no longer works and the maracas plays out of sync with the music.

Futurehit delivers both a maracas that sounds like you are playing it by shaking it and the MIDI works if you change the tempo.
The downside is that it is non-standard, hard to figure out, tricky to program and virtually impossible to play live.
We expect you to use the MIDI loops we provide. Anything else is supplementary exercises you undertake as an enthusiast.

> I really want to play these percussions on all levels of velocity on my own!

The 6 articulations of these two instruments are accessed via a velocity split of a single MIDI note. This is to not use up a lot of MIDI notes and is not related to futurehit.

>Is there a chance that there will be an option to deactivate futurehit in the future?

Not likely. Particularly not in EZdrummer. In SD2 you have access to the individual articulations and can MIDI map them as you like, bypassing the velocity split. Two of the articulations don’t use futurehit and can be played live.