I think it would be very useful to have the possibility of applying the "Amplify" function to "Zxy" values, as they are for VSTi general volume, modulation wheel, expression etc, and it would be very handful being able to amplify those values, same way as we do with non-midi volumes...
There are shortcuts for adding values to effect parameters, maybe there should be some for multiplying as well. MIDI Macros are universal and not necessarily volume-related, so it would be very counter-intuitive and confusing (and often even unwanted) to let the Amplify function modify them too.
Also, wouldn't it be nice to have the possibility of adding several effects columns in a same channel, instead of creating, for instance, new channels for each Zxy effects we want to use simultaneously (like "general volmum" + "Expression" + "Modulation Wheel" etc) ?
Yes it would, but it would also be an insane amount of work and would require a lot of thought to be put into pattern commands that was so far not necessary. For example, what happens if two contradictory commands appear in the same pattern cell?
A better and easier to implement idea are plugin channels which could be configured to belong to a specific plugin and then you could for example put any kind of MIDI CC into them. It has been considered before but is also quite a lot of work, especially in the pattern drawing code.