Im thinking the acrocanthosaurus would be a good new creature. It could be bigger than a rex but smaller than the giga, its saddle would fit up to 4 people 3 of wich could use guns, it would alao have 30% damage reduction and a bleed attack that would last for 5 seconds. It could also have the ability to gain aggro like the shadowmane then do a large head smash that would do more damage than a normal bite attack, it would also have a tail swing and a rage mode if it ate a certain material. It would be a good swimmer and would be found in the redwood forest, the swamp, and the desert, it would also be slower than a spino but faster than a rex. It would be tamed passivley like a troodon but it would need to eat less to tame.