Flash memory, 128 Mb user Memory,audio synchronization with wave/Mp3 files,touch screen,better drums,editable home page (like technics did in the past),appregiator from the XF synth,easy to edit and to make custom style.s (see old Technics),Hdmi output, faster system processor, Digital output Spdif (5.1 surround editable panning ,positioning)
SD card slot,XLR output/input,better vocalist,
Leslie effect with "stop"function and editable.

Just a few ideas for T4/T5

Impuls/Soryt
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Genos2,Korg Pa5X , Yamaha YC61, Ventilator2 . : https://www.youtube.com/channel/UCmC6hdAR1v5lYN8twfn0YbA