I prefer to work in the realm of a haunted hayride and I have years of experience with haunted hayrides, a few quick thoughts I have regarding your questions are:
I believe that there are a number of scares that can work well on a hayride, however, as with all scares, regardless of the setting, in my opinion, it boils down to this; to what extent are you willing to go to in order to get a good quality scare? In other words, you'll get out of it, what you put into it. if you want a hayride that doesn't choose to go out and create big sets or builds, or go into the finer details or add pneumatics/animatronics or any number of other effects, or quality costuming and good actor instruction, then you've just limited your hayride and the results it will or could bring.* So the answer is yes, there are scares that will not only work very well for hayrides but many of them are also ideal for hayrides, as long as you are willing to invest into them.
No matter how big or detailed a hayride might be, it can be difficult to scare 20 to 30 people on a hayride for a few reasons, two of those top reasons are simple, first: there is strength in numbers. Secondly: no two people are alike. Now add in to the mix that a hayride lacks what many walk through haunts offer, in which its guests must often navigate their way through pitch black, close quarters for an overall feeling of claustrophobia, while brushing past creepy characters, which of course a hayride would not offer any of those indoor advantages, however, a hayride in autumn out on a dark trail has its own advantages that simply can not be replicated indoors, so therefore it's smart to use those things to your benefit and play to that strength. My thought when I design sets/scenes for a hayride is this; I know I can't scare them all but I can entertain them all!
As for the raised loading area, I have worked with and without a raised loading dock (as I call it) and based on both of my experiences, I will take a raised loading dock every time and here's why I say that; A raised loading dock eliminates some negatives or concerns right from the start, a raised loading dock, with a plank like system and hand rail seems to give the guests a more comfortable, safer and may I say, a more even way to board the wagon. Without a raised loading dock, I have seen people run around the wagon throwing caution to the wind, I've seen some slip and fall from moist ground in an over zealousness to get the "best seat." With their enthusiasm to board, I have even seen some run in between the wagon and tractor, hopping/tripping over the hitch (put a stop to that immediately!)* So in my opinion, a raised loading dock offers a safer, quicker and surer way to get your guests boarded and situated. I also recommend a raised dock for disembarkation.
In regards to a covered wagon, fire, electricity and so on, I would be happy to offer my thoughts on all, So feel free to send me a message if you'd like.
I want to mention that I have a few hayride themes which might be called, true productions. I have spent endless hours in developing, the story line, scares, characters and more, all of which are truly original and unique, very detailed with the target goal being to make the hayride guests feel as though they traversed through a movie that was taking place around them! It is my hope to make these hayride themes a reality, so if you might have an interest in this, I'd welcome the chance to speak further with you about that as well. Best of luck with your hayride endeavors.