The topic of human stem cell is arguably one of the richest topics in modern biology. With its vast amount of potential uses and relative novelty in biology, it provides upcoming biologists with a wealth of possible topics to investigate. Since you are writing a thesis in academia (as opposed to a report in industry), you have the great advantage to bypass patents and research stem cells without charge. Some specific topics in human stem cell technology are cell transplanting, cell manipulation and electroporation.
Conservation biology is a relatively new field that investigates methods of preserving the earth's biodiversity. This area is multidisciplinary, and therefore allows researchers to use the knowledge of other fields in its inquiries. You too can create an interesting thesis topic in this hot field of biology by reaching into other fields and finding their connections to Earth's biodiversity. Some thesis topics are game theory's application to biological systems, economic models of biodiversity and conservation policy-making.
Although botanists and zoologists have identified most species of animal and plant, the majority of fungi remain unidentified. Not only are individual species of fungi of interest to biologists, but their role in ecosystems is also noteworthy. Thesis topics on fungi include the economic importance of fungi's role in ecosystems, fungi as a health product for humans and investigations of unidentified fungi.
When most think of biology, they do not think of it as a mathematical science. However, the field of mathematical biology sees mathematics as an important tool that biologists can apply in the form of equations, formulas and models to solve biological problems. Today, with the large amount of unanalyzed data sets, those biologists with strong mathematical backgrounds have an exceptional opportunity to apply their mathematics knowledge to bring something to the field. A thesis in mathematical biology should apply math or statistics to solving a problem in biology. Examples are using mathematical methods to analyze the human genome, applying nonlinear dynamics (chaos theory) to biological systems and using Monte Carlo methods to simulate ecosystems.