I have come across a number of personality theories. There are even disciplines of personality theories, dividing these theories according to the assumptions they are based upon. The most accepted/used theories seem to be Big 5 and Myers–Briggs Type Indicator (MBTI). However, I assume these theories use traits to determine people's personality types too.
For example, the Big 5 traits seem to be of very limited scope. Also, measuring traits like introversion and extroversion using some random questions which have polar opposite choices seems to be a fitting into a box sort of thing. There seems to be no scientific method while testing to determine whether people fall into one region or other for all the defined traits.
By scientific determination, I mean one could use methods like the one laid out in "Cerebral Blood Flow and personality: A Positron Emission Tomography Study" to determine between, say, introversion and extroversion traits. I am not sure whether the other traits have such methods researched enough to distinguish between them.
Are there any personality theories that are based purely on scientific grounds?
Are there any existing personality theories (that are being researched) that could one day be backed up by scientific research? I think the Big 5 might come under this category, but any evidence to the above regarding any theory will be appreciated. Thanks!