I think the biggest problem might be whether there is room for four vehicles abreast, plus graphics for the halt itself, on a single tile. If there isn't, and one must take all major pak sets into consideration, it will look ugly without a massive change that will likely never happen. If four vehicles do fit, making vehicles pull over to the right (assuming drive on right here) to stop at a halt shouldn't be any more difficult than overtaking. But it takes someone with the time and motivation to do it. Since there isn't much traffic in Simutrans beyond your own buses, most vehicles passing a halt will likely stop there (at least in my games), making such a feature less pressing. For big bus terminals, using loading bays (or bus bays if the pak set provides it) are an not unrealistic alternative.