You truly shine when it comes to making the characters your own, while also keeping them very close to their canonical design. It is a difficult balance to strike, but you do it every time! So here, you have already done a fine job.
In giving up the Adventure Time characters' recognizable flatness and wiriness, you allow yourself to indulge in the texture and layers of color that they are capable of exhibiting. Fionna's hair and the airy, soft, bubbly background showcase your coloring the best here. Gumball's hair and Fionna's cheeks show off some delicate shine, and the lighting seen here is reflected pretty well in the overall composition, too!
You also were not afraid to add an eyebrow to Fionna--one line that I feel creates her entire expression, one truly fit for a tomboyish heroine
The composition itself is nice as well--classic, and a nice homage to the Sailor Moon, girl-heroine archetype so lovingly borrowed from in the genderswapped version of Adventure Time. I would suggest adding a bit more depth to it to increase its impact on the viewer--make some of the bubbles appear farther away, for instance. As it stands, it is a static, but very pretty and eye-catching portrait of the characters.
Final suggestions would be a bit more depth and detail in Fionna's skirt and some refining on the fingers--namely, PG's around the lollipop Awesome work!
You are so welcome!
By the way, how did ya New Year Eve's went out?