Here is the problem... what your students need to work on, to get the most improvement is their basics. But, the basics are the last thing your students want to work on, once they think they have it.
I like to look at my students, and find something basic that they all need to work on. Every class, we will work on that for a few minutes before doing the "fun" stuff. Over time, that basic will improve, and so will a bunch of other things as well. (heck, you will improve in that basic skill)
You don't have to spend a ton of time on it, 10-15 minutes per class should do it, over time. Consistency is key.
You can also disguise it a little. Say you want them to work on the lunge punch. You can just do lunge punches. You can do them super slow, as a warm up stretching exercise. You can do them fast, for speed or cardio if you do a lot. You can put them into simple drills: three in a row, block then punch, block then three in a row. You don't have to get complicated. You can cheat, and "work on blocking the punch." The class is now focused on the block, but they are getting their reps on the same punch. You can do blocks, parries, counter attacks, take downs whatever, as long as you are using the same punch as the attack. In their minds, they are not doing the same thing. You can do combos, that start or end with the punch... again, don't get over complicated. The trick is to change what they think they are doing, while they get a ton of reps on the basic.
After a few months, you should see improvement in that basic technique you choose, but also in a few other areas as well. Then pick another basic. Remember, the basics are the basics, because the rest of the art is built on top of them. If you improve the foundation, you improve the whole building.