First and foremost: In-depth documentation of the toolkit. Literally today, after 2 weeks of modding, after making a lovely level and coming up with a story for it I became discouraged by a road block in form of lack of proper documentation how to set scripts right and where so that they would compile without errors (Your own scripts, not mine) like those saying that "character player is not defined" in one of the main game wall-of-text scripts.
For me using DOS1 engine is like trying to drive a car and having to call different people to ask them how to do it and them telling me that very few of them actually know how to drive this car. Or like needing to be an IT engineer in order to use the SatNav.
I hope you know what I mean.
I hope I am making any sense.