Do you agree with three assumptions made in CAPM? However, what if those assumptions are changed? What happens if investors have different expectations? What if the market is not competitive? You can imagine a plenty of variations from the benchmark cases, and all the academic papers have been developed up to now starting with these modifications. You do the same exercises, and take the following steps:

1) Pick one part you want to change among the three assumptions (you can find many assumed even in one sentence!)

2) Describe outcomes of this model if you put more realistic components. To this end, you can refer the model recently developed. Please search scholar.google.com or articles which cited mean-variance portfolio theory of Markowitz, Sharpe or more so. For instance, if you click the link cited by 45205 in Markowitzs portfolio selection article, you can find many different versions of the model.