I haven't made ATCs in quite a long time, but the link below might just inspire me to start a new series... this squidoo page has lists of themes, colors, and techniques for each letter of the alphabet... ATCs, postcards, journal pages, you could do anything with these lists as a jumping point of inspiration--
A: alcohol inks or acetate?
B: beeswax or bubble wrap??
C: crayon resist or chipboard?
D: dry embossing, or doodling??
E-Z... just as many great ideas!
...so many ideas... so little space in my art studio!! lol!!