The Yunnan region, in South-West China was very close to the French authorities. They even built a railway between there and Vietnam. The region was often at odds with the central Chinese government and the post-1880 period was not the best for Chinese authority anyway.
https://en.wikipedia.org/wiki/Kunming–Hai_Phong_Railway
It's not out of the field of possible for a Yunnan governor to say fuck it and just declare itself independant. After a couple years (necessary cooldown period), it asks for French protection.
The other possibility is heavier French involvement in the region, especially economic. At some point, the French ask for a concession in Yunnan to "protect their economic interest".
They can pull a similar trick in the Canton region as it's contiguous to their Protectorate of Tonkin. The Brits might not appreciate but I'm sure they can be placated in other ways. Of course, effective control of the region will be VERY tenuous. They already had massive issues holding on the Fort Bayard concession in Kwang-Tchéou, but cooptation of local elites through massive opium fueled bribes might help.
I guess they could probably annex the island of Hainan as it's litterally RIGHT THERE. Looking at maps it always annoyed me.
You probably cannot do both because of the Brits but there is some expansion possible in Thailand. I mean further expansion. Sure the Thai were not happy about continuous French encroachment but they couldn't do a thing, except ask the English who took territories as payment for their "protection".
The other area of expansion is South America, thanks to one of my favourite colonial stories: the Republic of Counani
https://en.wikipedia.org/wiki/Republic_of_Independent_Guyana
The Guyane border was defined, partially, by the treaty of Utrecht but it was always a bit fuzzy, especially because it's deep jungles. Basically the said a river was the frontier but Brasil and France couldn't agree which river it was. In 1886, a couple guys decide to try their hand at exploiting the territory.
It goes well, makes them a lot of cash. They see that a large local population actively wants to be administered by the French. Why? Well, they were former slaves and slavery was abolished in France. So our two guys decide to create an independant republic, under the protection of France. It worked quite well for a few years but after that, they couldn't get the necessary backing to exploit it more and France wasn't willing to annex and have a war with Brasil. The thing is, it would have massively increased the size of Guyane, just look at it!
Now, what if France had been willing to fight for it, or had another reason to fight Brasil?