After there being answers, I would like to give my own, rather generic and abstract perspective on this.
What are combinations done for?
Combinations involve a twofold effect for a single goal: They both keep the opponent busy and limit their own tactical options as they do not let them gain initiative and they call for the defense of one attack while opening the guard for another. The latter also involves the establishment of expectations regarding what comes next. Both aspects serve the goal of setting the opponent up for a hit that is able to penetrate their defense.
How can we transfer this insight into kick-only combinations?
Combinations work if they open a hole in the opponent's defense. For that, we can vary height, angle, and side of the attacks. It does not matter whether we actually carry through each attack or use feints, the important part is that our opponent reacts in a certain way.
For example, lets assume we do right roundhouse, right roundhouse, left roundhouse, front teep two or three times. This does two things: It attacks from different sides and angles, thus forces our opponent to shift their guard. Sometimes, for inexperienced fighters, this suffices to open a hole in their defense. But it also sets what is called a psychological anchor, ie. it establishes a certain rhythm and pattern that is expected. Now, if we suddenly break this pattern in the 3rd, or 5th time, like for example using the question mark kick instead of a front teep at the end, or a third right roundhouse instead of a left, there is a high chance our opponent will half-automatically follow the same rhythm and pattern as before, ie. they will fall for the trap.
You could just as well vary your combinations each time, making yourself completely unpredictable. But truth be told, this a) is way more cognitively demanding for you and will probably lead to hesitation in between which lowers the effectiveness of your combination and b) is less effective the more experienced your opponent is. Accordingly, you best go with 2-4 presets that you have trained beforehand and vary the number of consecutive attacks with the same preset with occasional surprise variations like just described.
Long story short, the expectation is to use variations in directions, angles, rhythm, and technique to set your opponent up without letting them gain initiative in between. They should be busy and not know what comes next since that is what extends their reaction times. The options are limitless, even with this limited set of techniques.