It has to be the same size as the 810, but a bigger screen ;4.3 or bigger. Less metal around the edges.
Same resolution is cool, but higher is even better.
Most important is processor --a much more powerful processor. The omap 3430/3440 is cool.
4+ row qwerty keyboard, even cheap phones like the Rant have 4 rows.
Don't necessarily have to have stereo speakers, but it needs a louder speaker --twice as loud.
Memory is fine with the 810. Only will change it to microSD only because it uses less energy.
A decent D pad.
More battery life.