I actually whipped up varients for Aasimar/Tiefling/Genasi that changed the way they worked a bit.
The *main* strength of an Aasimar lies in the Outsiderness and the Resistances, and the stat bonuses, not in the spell. So to create the 'low' Aasimar, at +0ECL, I ditched the resitances and balanced the stat bonuses (+2 Wis, -2 Cha), and it's OK.
You could also do a 'high' Aasimar that had an additional resistance, a +2 to Str, and Daylight instead of Light, at +2 ECL.
Or do a tiefling who was 'charming,': +2 Cha, +2 Int, -2 Con; same resistances, Charm Person instead of Darkness, etc.
Or do a 'water' Genasi who actually was oozey, etc.
I can put these up in a thread in House Rules, if people are interested (with the Life and Death genasi, and the Neutral Planetouched).