Hopefully the investigation was successful (otherwise, why didn't you do something about it earlier on!). Future improvements could come from having better data. Maybe more, or a wider range, or looking at an additional aspect. For example, if looking at children's language acquisition your conclusions would be stronger if you doubled the number of children studied, or maybe recorded them in different environments. Maybe you noticed a difference in one child who had older siblings and if done again you would include others in this category to see if this impacted on their development?
There is an understanding by the moderator that you are limited by time and word-count so cannot produce a full academic study. The review is for you to show that if given more resources, you could use what you have learned this time to produce a better study next time.