The phrase "a slightly more complicated" is correct and usable in written English.
You can use it when comparing the complexity of two or more things, indicating that one is a bit more complex than the other.
Example: "The first problem was straightforward, but the second one is a slightly more complicated challenge that requires deeper analysis."
Alternatives: "somewhat more complex" or "a bit more intricate".
Here's a slightly more complicated one.
That El Mozote in late 1981 was not a guerrilla town is a fact central to Rufina's story and lies at the heart of the mystery of what happened there; and though it is a fact -- one that almost everyone from the zone affirms -- it seems to have nonetheless been a slightly more complicated fact than Rufina makes out.
While it did only launch 12 weeks ago, IntoNow did have a slightly more complicated beginning.
A look at the company's vitals paints a slightly more complicated picture.
As of 2013, getting a visa became slightly more complicated.
Built-in programs like the Windows Files and Settings Transfer Wizard or the Mac OS X Setup Assistant can shift music files from the old machine to the new one on the same operating system over a cable connection, but it is slightly more complicated to make a move from a Mac to a PC or vice versa.
However, if a bowler hits a spare or a strike, the rules get slightly more complicated.
The case of a tandem duplication is slightly more complicated.
In an insulator, things are slightly more complicated.
In the summer of 1964, after the shooting of a 15-year-old by an off-duty police officer touched off riots in Harlem, Mayor Robert F. Wagner invited Dr. King to New York on a peace mission (one made slightly more complicated by the fact that some black leaders resented that the mayor had invited Dr. King without consulting them).
It generally relies on human-defined classifiers or "feature extractors" that can be as simple as a linear regression, or the slightly more complicated "Bag of Words" analysis technique that made email SPAM filters possible back in the late 1980's.
