Hi Zikomo,
You could safely add any of the substitutions you mentioned, or omit the coconut entirely - it's far from necessary in this or any other base sauce recipe. If you were cooking a chicken Ceylon curry then yeah, you need some coconut flavour from somewhere, but for a base gravy the most Important flavours are very well boiled onions and turmeric. Other additions like carrots, red peppers, tomato, garlic and spices only add small changes to the flavour.
Most of the flavour in your finished curry will come from the garlic, ginger, spices and tomato paste you fry at the beginning of the dish. The base gravy will just be adding bulk to the sauce. Don't get too hung up on trying to replicate exact ingredients in a base recipe. Most important is that it's mostly comprised of onions, has turmeric in, and is cooked for long enough to give a mild flavour without a harsh bitter onion taste.