I have used Keurigs in office for ages. Never had luck with DIY as people leave a big mess. But it might be fun to do cinnamon as you suggested, pumpkin spice, instant hot chocolate, pulverized vanilla bean or hazelnuts, ground pecans and then add honey instead of sugar, caramel candy, peppermint candy (or stick a stick in the coffee) ...
Hmmmmm yum!