In my opinion , you should select a filed ,such as RF , Data convert , PLL.
etc, and the RF is too difficutl if you have good models and software for design ,and the testing is diffiult too.
ADC such as sigma-delta is very interesting and many sigma-delta theory and circuit theory used in it.
PLL is used widely , such as RF , Digtila system etc, and the noise performance is the key problem ,how to simulate and make it low is a good topic.
Hope this is useful for you