You may need to first do desk research on development finance in broad term firstly Then you can narrow down to some specific subject areas of development finance e.g poverty, financial inclusion, microfinance, MSME finance, rural finance, development finance institutions, informal finance etc. You can establish the current debates a in any of these areas and identify the gap/missing links in such debates. You can then come up with your own topic that will satisfy the missing link .
Blended finance is an increasingly popular topic (i.e. leveraging private finance with public funds to multiple impact) - not a particularly new idea, but there are niche business areas (particularly those with higher risk to private investors eg. agriculture) where there is scope for research to identify challenges and solutions to overcome these.
'Development' is so broad, and I don't know where you interest lies. There is also interesting work in using blending for social investment, eg. in healthcare provision.
You may also look at "Financial Sector Development, Market driven or institution driven". look at a 2015 Working paper by IMF, "Rethinking financial Sector Development" for clearer direction.