As a recent migrant from Handcent, I'm loving having discovered Textra.
The offered colors under 'Defaults for Bubble Colors' are fun, but every color (except white) ties the font color to white. A big improvement, for me, would be adding (say) four pale colors with the associated font color in black.
That's something that, for me at least, would add a lot and maybe wouldn't be tricky to incorporate.