First I want to explain that we as a species can and at some point will provide a place for this to happen, through terraforming and genetic recreation give these creatures lost to extinction a viable and fundamentally perfect place to exist, with that said lets begin after we have Terraformed and recreated these creatures.
We begin by genetically modifying a plant to produce a substance that resembles and tastes much like bacon. Everyone loves bacon and I am sure the flavor would please even a Tyrannosaurus Rex. This plant would begin by producing very large pods that would drop all year round a fruit that gives high volumes of proteins, vitamins and minerals. It may even be a modification of the Jack fruit and includes a edible encasement for when the pod drops to the ground. This would provide all that this creature needs to survive happily.
On this planetoid we would plant in large quantity these plants as well as others, but only after this fruit has taken its place, this ensures long lasting superiority as to maintain the ever increasing growth and population of the animals therein. Then large trees such as the sequoia tree would be used to provide shade and the occasional scratching post. This would begin the companion seed system and we would be used to provide the necessary minerals for healthy sustainable plant growth over the many generations to come.
Now that we have planted the proper sustainable Bacon-Jack Fruit, we can begin to introduce animals, considering we have decided on the T-Rex we should make sure not to introduce anything that would provide large sources of food, or competition for space or resources. These creatures need to be agile and match or provide greater land speeds then the T-Rex, since the T-rex has a land speed of 17 mph we can conclude a creature such as a Gazelle with a land speed of 60 mph with advanced agility would be a good fit, these creature would graze upon the grasslands and not impede the space or provide a alpha type competitiveness nor would it be easy prey. We do not want to give the T-Rex an optional food source as we move it towards plant-based consumption.
While we begin the transition to plant-based foods we must understand the T-Rex will be not always as large as its adult form and this would make the Gazelle a good looking choice, so we provide the Bacon flavored pods all year round through modification. This fruit does not run away, nor will its seeds dissolve n the stomach and this will give the T-Rex a place within its circle of life to spread with added fertilizer the droppings of dung and ability to spread as well. The Gazelle is weak as a baby or new born and so we must provide an even easier meal with a high quantity of value to the T-Rex.
It should not be conditionally difficult for a T-Rex baby to consume and this can be provided by making its interior soft and the layer outside easily torn by its mother or father. This would make it less likely that a Gazelle would become a target. As with all creatures there is a curiosity we must deal with and so make the surface area of the planet contain various outcroppings of shelter a Gazelle can hide or sleep in which protect it from the T-Rex would be rather simple to produce. Understanding the ability to graze and eat is now easier for both creatures to do even side by side gives us a very high chance of perfect co-existence.
Over the first few integrations of creatures we may see some destruction or attacks attempted on the Gazelle though this is a learning process for the T-Rex and even as a young baby as long as a save area is provided such as Tall grass for cover and rock outcroppings it should be unlikely a T-Rex which notices movement would see a aby Gazelle sitting still. We will have issues with the possibility of randomly stepping upon the Gazelle, which is why we provide rock outcroppings as the T-Rex will likely walk around the area rather then weave between it.
While the synapses of the T-Rex over the first few generations will still likely provide a desire to eat living creatures, the use of energy to chase them and the lack of energy given from each will be far less. We can ensure this happens by making the Bacon Flavored plant fruit is larger then the Gazelle itself, each fruit would be similar and obviously in its growth less in size at different stages of its development. The key here is to allow the synapses that through evolution learned to desire meat and chase the opportunity to discharge naturally, over the generations the T-Rex will gain satisfaction from eating the fruit, more proteins, vitamins, and minerals and through this its brain will learn to seek the fruit instead of the meat. It will be a process but by providing each pathway a solution that directs it towards the fruit we can educate the creature to adhere to sanctity of all life.
While the T-Rex will have no competition ensuring that they do not eat each other is important, chances are they will only quarrel for mating rights. We will make fruit so easy to obtain that fighting over food becomes non-existent and until the population exceeds this there will e no issue, it will be easier to keep going then risk lethal force or injury to stay. When the T-Rex passes away the cycle is complete as there is no other meat eater on the planet and the body is given in its entirety back to the planet and the plants for nutrients. When this process is complete we can then introduce any number of companion based animals as long as they are of similar design and follow the same construct of the Gazelle such as deer, or antelopes. The land speed must be greater and they must be capable of living and co-existing in the same way.
The plant which we design must be capable just as the Sequoia, to grow to ever increasing size and longevity of life to provide enough sustainable fruit as may be needed, perhaps not as long as the Sequoia but with enough size that larger structures can exist, but also not so long that nothing else grows. The return of plant growth decay also gives nutrients and minerals back into the ground through a natural compost.
Each process should be analyzed properly before to ensure proper placement and companionism is validated. The pathways of each creature could be instructed and provided with a complete ecosystem for existence along the pathway to sanctity of all life. There are no creatures this cannot be done with. We can all have our paradise.