Hmm...
Mythical Logias? Logias that are fantasy elements? Like a Mithril, or Adamantium, or Kryptonite Logia? You'd have to make them "legally distinct", but you could get more creative effects that way.
Maybe play up the environment changing effects more? Like, Logia Awakenings not just effecting the rocks and stones of a place, but changing the plants and animals around them, too. Like, Akainu's Magma Fruit changing a random house cat that happened to be passing by into a Lava-cat that he could command? Flowers with lava-pollen? Turn the environment around them into a completely alien world than what existed around them beforehand. Make it a true "environment", instead of just changing the land.
Have different Logia users work together? That way we could have more "chemical reactions" take place? Like, have a Mentos Logia work with a Soda Logia so they could shoot giant streams of high-powered liquid at opponents.
Make the Logia users lose control a little more? Like, maybe turning your ENTIRE body into your element changes your psychology. You start thinking differently when you're in a fully-elemental state? You start thinking more like the element. Having this huge amount of power being offset by the possibility of losing who you are, if you go too far using it. It would at least make it more of a tug-of-war internally, for the Logia users.
Maybe make the Logias more susceptible to their subconscious desires? Like, it changes the environment around them, but not always the way they expect. Maybe it changes the twists the environment in ways that bring up long lost memories or suppressed desires. It turns the environment more into a "dream realm" that is their mind trying to work through some sort of trauma in their past. A different way to play up the psychological element, basically.
Let the Logia users be able to forge weapons and armor that other people can use? Make it so that they can make and sell swords or armor made out of Lava and Light that other people can use. Make them weapons manufacturers for other villains. Put them in more of a executive role. Logia users that can make their element more usable for everyday thugs. That way, it completely upsets the power balance of a country.
Make it so that they don't just change the environment, they can "absorb" all of their element in the environment, too. Like, make it so that Sabo can absorb all the fire or heat in a area, so that no one else can use guns, or fire of any kind while he's around. Have Kizaru be able to absorb all the light in the area around him so that they have to fight in total darkness. The Logia user becomes the only source of the element in that area. Changing the environment not because it turns INTO that element, but because of the LACK of that element.
Maybe let the Logia users alter their states of matter more? Like, no matter what the Logia element is, you could still switch between a liquid, gas, and solid version of that element. With the highest level being the "plasma" or "energy" form of that element. It would give them more forms, like Chopper. And you could get creative with coming up with a "liquid" or "gas" form for a element like "light".