Short Answer: Elle's continued ascent
Let us pause to appreciate what an achievement the Legally Blonde series is. In a world in which a franchise, to be successful, seems to need to be an action or special-effects driven poperty, this little comedy that could be getting a Part 3, and already had a spin-off introducing new characters (Elle's cousins) AND a musical version (of the original film).
(So... where IS Part 3? Well, it was scheduled for 2020, but then we had that pandemic...)
Along with the success of the so-far three-part Pitch Perfect series (not a stage musical yet-- really?), this should prove that comedies-- including ones with women protagonists-- can do just fine, thanks.
OK, but where does the Legally Blonde series go next? It depends on the faith it has in itself.
If it it has a lot, then Part 3 has Elle starting her own firm, and then Part 4 has her running for Congress.
If it doesn't, then Part 3 starts with Elle already as the head of a successful law firm... who decides to run for Congress. See, at the end of Part II, she glances at the White House and winks. But we can't have her run for president just yet...
Since the movies thus far have hinged on Elle's knowledge of fashion (the first one) and her love of animals that drives her to fight testing cosmetics on animals (the second one), then the next movie should deal with one of two other major issues that mar the fashion industry.
One is that many clothes are made, still, with sweatshop labor. If your T-shirt costs $10, then everyone who dealt with that shirt on its way to you made less than $10 per T-shirt... right?
As awful as that is, it may be too awful to contemplate for a lighthearted comedy.
Which means the movie should deal with the other issue: pollution. Making clothes, especially the production and dyeing of fabric, is awful for the environment. Rivers downstream from jeans plants run denim blue.
In the first film, Elle had to confront a fitness guru she was a fan of for lying about how she lost weight. In the second, she had to confront a cosmetics company she was a fan of for testing on animals. This time she should confront a clothing designer she is a fan of for polluting... say a beach she loves to frequent.
In the start of the movie, Elle is running this law firm specializing in fashion-related clients. One major case is about the plagiarism of a design, perhaps.
In any case, she sees that her favorite beach is polluted. Maybe she has her dog swim the water-- "Wow! The water so beautiful! I have never seen it so... blue!"-- but he emerges dyed blue!! She starts to go all Erin Brockovich and traces the source to the clothes-dyeing plant, which she then traces back to her fave designer.
Her passionate drive to clean up her beach and punish the polluters (and her blue dog) get her headlines, and she is scouted by a political party to run for Congress. She goes up against the incumbent, who is funded by the dye plant, and everyone underestimates her (again), to their detriment (again).
Oh, stupid people... when will you ever learn never to bet against blonde?
In the one after this, we start will Elle in the Senate, and she runs for president.