Golball
If this is a paper excercise or a thesis project then the sky is the limit at what could be done.
However, the reality in this modern world of certification standards, etc. mean that you are looking a lots of dosh to see things through.
The easiest way to improve performance is to reduce weight, or add power, no way to do this cheaply in a certified design.
In the light-sport/experimental field however there are many good things happening. If you take a look at the classic piper cub-type aircraft for instance, there are a few manufacturers here in the US who are offering their version of a modern "cub", with modern engines, and/or carbon-fibre parts in the structure.
There is an article in the recent Flight magazine about one of these,
http://www.flyingmag dot com/aircraft/lsa/sport/carbon-cub-ss
or you could check out one of the manufacturers website at the following;
http://www dot cubcrafters dot com
I still regard my initial 12 hours of piloting in a 1946 J-3 as being the most fun I ever had while flying.
Cheers,
f