How will London's role as a financial center be affected after the Brexit and what are possible alternatives?
London remained for long as a key financial centre for Europe due to several reasons:


Historic: several universities and authors dedicated to the study of wealth (Smith, Ricardo, Malthus, Keynes
Combination of privileged international positions with US (language + colonization) and commonwealth countries, as well as an average sized internal market
Looser regulation than most countries in EU, providing lower barriers to entrepreneurship
Fairly efficient justice system
Management and investor attitude: social class or country of origin is less of an obstacle than in France, investors not as conservative as in Germany


Comparing these factors to other large cities in Europe, it will be hard to match it. 

In Germany, Munich or Berlin could be more attractive for investors than Frankfurt. Paris would be the favorite in France, but regulation doesn't attract investors nor entrepreneurs - so Amsterdam or Luxemburg would be much more inviting in this regard.