The idea behind the Trapdoor was to add an element of luck to the terrain.
I really don't like the idea that a nible gladiator can be just pushed towards a pit and then removed from the game. I think that if I was being pushed towards a hole of certain death I would at least try and jump it. Adding the doors to the pit gave the ability to play that option of "chance your luck". Hence whay the bigger Gladiators don't get this option.
As you say a hidden trapdoor would be almost impossible to play and far too complicated.
Maybe another Reaction Rule could be Leap of Faith and a roll to aviod the obstacle? Much more specific than React: Movement.
As for the Fire, I initially was going to treat this as a Fire Pit,. However, as I don't agree with the fatal nature of Pits, as explained above, I set the damage at 3 as being burned hurts just as much as being pushed into a spike wall I would say (okay I haven't actually tested it but both would bloody hurt) and both would have a lasting impact on the Gladiator. But the risk of catching fire on an oiled linen clad gladiator is still there and fire would have to also provide a fatality threat. Hence the Luck roll.
I will probably adopt these as houserules anyway but am always open to suggestions for ways to better them.