If you want a special key "Next TextField" above standard keyboard of all TextFields of a view
You have to design your own button (size, border, title, image, ...).
Add one call to your code and it is done

import ui
from objc_util import *

def NextFieldKeyInTextFieldKeyboard(view, button, background_color='lightgray'):
    # parameters: view = main view of textfields
    #                           button = ui.Button designed by user (size, title, image...)
    #                           background_color = background_color of InputAccessoryView
    # create ui.View for InputAccessoryView above keyboard
    v = ui.View()                                                   # view above keyboard
    v.background_color = background_color   # background_color of user choice
    v.width = ui.get_screen_size()[0]           # width of screen, thus of keyboard
    v.height = button.height + 4                    # only for Next button

    # code executed when Next key is tapped
    def Next_tapped(sender):
        s = sender.objc_instance.firstResponder()._nextKeyResponder().becomeFirstResponder()    
    button.action = Next_tapped                     # code to be executed if button tapped
    v.add_subview(button)                                   # set as subview of view

    vo = ObjCInstance(v)                                    # get ObjectiveC object of v
    # attach to each textfield an InputAccessoryView abobe the keyboard
    for sv in view.subviews:                            # loop on all subviews of main view
        if 'TextField' in str(type(sv)):        # subview is a TextField
            tfo = ObjCInstance(sv).textField()# get ObjectiveC object of sv
            tfo.setInputAccessoryView_(vo)      # attach our accessory to textfield

if __name__ == '__main__':
    main_view = ui.View()
    main_view.name = 'NextFieldKeyInTextFieldKeyboard'
    main_view.frame = (0,0,280,140)
    main_view.add_subview(ui.TextField(frame=(10,  2,260,32)))
    main_view.add_subview(ui.TextField(frame=(10, 36,260,32)))
    main_view.add_subview(ui.TextField(frame=(10, 70,260,32)))
    main_view.add_subview(ui.TextField(frame=(10,104,260,32)))
    button = ui.Button()                                # Button for Next key
    button.frame = (0,0,32,32)
    button.font = ('<System-Bold>',24)
    button.title = '⏯'                                  #  emoji as title
    NextFieldKeyInTextFieldKeyboard(main_view,button)#,background_color='green')
    main_view.present('sheet')