One substantial way of improving Simutrans-Standard might well be to improve the documentation: make video tutorials, up to date step by step guides, indeed, even add an in-game tutorial mode. One worthwhile new feature that wouldn't add much complexity is the separation of tunnel graphics and way graphics that was discussed previously. That is all dependant on developer time, of course, and that is often limited, but, developer time permitting, there should never be a shortage of worthwhile things to do.
If any developers have spare time and are wondering how to fill it, assistance on Simutrans-Experimental would be much appreciated, as there are plenty of projects there that could do with lots of work.
Of course, it may be that the developers, having reached that happy nirvana of having no outstanding projects, and a mature game, would rather spend their time playing their creation than working further, which would be quite understandable.