I usually build up full-screen abstract stuff using a combination of Add Noise with lots of various FX, and generators (such as gradient). Since lower thirds might want to match the look up the full-screen titles (I know the Juice ones do this), you can mask off a portion and maybe layer some "trim" on the edge so it doesn't just stop. Since the heavy FX can take time to compute, you should render these off as looping files that get used whereever you need them.
///d@
"Lower Thirds" is somewhat of a misnomer, but basically refers to having some sort of alternative display set into a portion of the screen. The most common example of this you have probably seen is the stock ticker or headlines or name of the person being interviewed shown at the bottom of the screen during the news (filling the lower third of the screen, so to speak). While these can be created with simple solid rectangular backgrounds, it's often nice to spice them up with some sort of logo or animated graphics.