Firstly I'd like to say what a superb app this is. What is most stunning about it is how FPSE is actually able to significantly improve on the original game in OpenGL mode.
Running in full HD with antialiasing and texture filtering enabled, Tomb Raider 3 looks more like a PS2-generation game than a PSOne gen, and Soul Reaver looks so good it almost gives the best non-emulated android games (e.g. Dead Space) a run for their money.
What I would like to suggest is that the developers try to go one step further in this direction by adding support for framebuffer-level (post rendering) effects, the most desirable of which is probably a bloom lighting (HDR) effect. My understanding is that there is hardware support for this in OpenGL ES (though not necessarily hardware support on all phone GPUs - I have no idea), though I would imagine that even without hardware support a software implementation of bloom lighting might be viable.
Other framebuffer-level effects I am aware of are screen-space ambient occlusion, and HSL colour correction (see QEffects GL at https://code.google.com/p/qeffects-gl/ for an example of what is possible with this type of effect), although of course support for just one such effect (e.g. bloom lighting) would be a fantastic starting point and proof of concept.
Keep up the great work