Concept A - Only shoulder animals are tamable. Can even make it so they are the only animals on the island. This would make PvP interesting as you have small armies. Reduce the breeding and hatching time as some of them, like Compy, take almost 4 days to mature.
Concept B - Boosted rates, either everything or just XP, but have it on Hardcore mode. This can work with either PvP or PvE.
Concept C - My personal favorite as I am currently playing it on Single Player. Difficulty is 0.144 so maximum dino level is 30. Do what you want to balance the rest out, but I play with 5x tame, 40x maturation, and 0.025 imprinting so I can take care of stuff in one session as it does not run while I am offline. You can limit the dino level ups to 30 as well. This makes the game a very satisfying challenge. I played official for over a year, and it was no longer a challenge once you tamed a decent dino and had flak armor. All these Unofficial servers are super boosted and have higher level dinos. Super stats just make the game boring after a day, and higher level dinosaurs mean nothing if you can get high level gear quickly. If anyone is interested in my full list of changes (spent a month testing it) then feel free to PM me. I actually play this more than my Unofficial server.
Concept D - Solo tribe, no alliances, PvE, hardcore. This is the ultimate bragging rights server. Have it on 2x or 3x experience so players do not become demoralized if they get jumped by a Raptor or Purlovia. You can even remove saddles for large dinosaurs and above to keep up the difficulty a bit.
Concept E - NO FLIERS. This will force players to create their own caravans on the ground. This would be interesting for PvE and PvP. Dinosaurs like Gallimimus and Procoptodon will become relevant. Once players obtain flying dinosaurs, they do not even bother with the land ones unless it is necessary for something (like Ankylosaurus).
Concept F - Very slow resource respawn rate. This will force players to consider how they harvest wisely, and show the effects of deforestation and other detrimental effects to the environment. Resources will become very valuable. I can see players focusing on fiber and leather goods with this, and underwater exploitation becoming a hot commodity as there will be less competition. Possibly have to be PvP so you do not get trolls and to "protect" your personal resource hubs.