Hello, and welcome to the forum.
You have some good ideas for expansion of the SEQ concept but I am not sure exactly how you would go about implementing them.
The parts kit for the Quad IIC is not as extensive for the SEQ, and there is no waiting list for this or the SEQ PCB as far as I know. LCDs can range from cheap and generic to quite expensive; you should check the backlight polarity and interface type (Hitachi HD44780) before ordering.
It should be no trouble to build a simple control surface expansion on veroboard or even a custom PCB. There is a chain connection for additional shift registers on the main board which you can connect DIN and DOUT modules to in order to scan extra buttons or LEDs. Keep in mind that without (semi-complicated) matrix wiring the maximum number of inputs/outputs is 32 per module, you have 34 buttons in your design.
The panel could be easily remodelled for the extra functions. The case is always more work than the panel, it is worth trying to locate this part first.
The software side is where it gets tricky. I wouldn't expect anyone (TK) to program these changes for you as it is a single use-case feature set. It is also unlikely that everything can be run on the same core module due to memory considerations; additional routines may affect the timing performance. In my opinion, your best bet would be to use a keyboard program running on a separate core, even an 8-bit one. Then you can forward the MIDI into the SEQ for note editing.
The SEQ does have a step-edit (303ish) mode, but I think what you are describing is a different application, and one which you would have to program yourself. The current interface is very powerful and there are many layers for accents and so on. Notes can be added with a standard MIDI keyboard.
It is great that you are thinking about something different and applicable to you, but if you say that this is your first project I suggest that you start with a basic SEQ build and see how that goes first. It would be possible to build an expansion later if you found the current interface too limiting.