I remember this being suggested in the past, though it got denied. Even then, immense vouch.
FreeOP gets extremely monotonous in my opinion. Do the occassional build, chat with the very few people that are on, and not much more.
It may be my bias in favor of Pokémon speaking, but I feel Pixelmon makes you have more goals, even more than the SMP. Catching Pokémon, creating and trying different strategies against other people...
The champion idea also looks like a bit of a good incentive. Perhaps once you reach Champion you can get some rewards as well. Maybe the gym leaders could be switched up periodically.
For the events, maybe there could be some kind of drop party but with Pokémon, with some of them also being shiny. We could go for some unique tournaments, maybe Monotype tournaments (only teams of Pokémon that belong to a certain type), and, if it was possible, Hackmon tournaments (teams of Pokémon with random habilities and moves, even those not normally obtainable by the Pokémon, these could be provided by the staff team and removed after the event)
For survival Pixelmon, I definitely think there could be some kind of easy way to access certain biomes. No one wants to spend 7 hours searching for a biome a rare Pokémon spawns in.
Coins in events could be used to buy TMs, perhaps. Or maybe there could be IV/EV boosters. Perhaps items to spawn (not instantly capture) a random legendary, or a random Pokémon egg. There are also Pokémon with special textures in Pixelmon from what I've read, so those are also an option.
In creative mode you lose the excitement of actually finding and the Pokémon, since they are all at your disposal with commands. Sure, you can compete against others and still have the leaders and the champion, but I don't find it all as rewarding, in my opinion. And even if we had those, we'd have to be careful with teams that use illegal Pokémon if we wish to stay balanced. I doubt anyone would have fun fighting against a Pikachu with Wonder Guard and Air Balloon that also knows Rest. And I also doubt anyone would have an easy time tracking what moves/habilities are legal or not in each of the 800 Pokémon there are.
Creative mode Pixelmon sounds interesting, but is in my point of view not much of a good idea. I still think it'd be better than regular FreeOP, though.
In short, I think survival Pixelmon would be the best option, though I can't really come up with any original ideas for it at the moment.