Support large repositories!

"Moreover, merging between two different data sets is not solely a
technical problem, it requires artistic eye because even correctly merged
result of two great-looking changes may still look like shit."

This can very well be true for code also...doesn't meant that a merge tool
to help the process isn't/wouldn't be useful. But, naturally the right way
to support this would be to have your VCS support multiple merge tools via a
plugin mechanism.