As a thought, it seems really odd to have the Lite challenge lasting 2 months, but the "Serious Challenge" lasting 1 month. Could it be worth changing the format of the Lite Challenge and making it a monthly/fortnightly challenge?

If you wanted to include a "Lite Challenge" prize logo that was drastically different, I'm for that. But maybe the Lite Challenge should become stuff like the OPDC.

Temple or Religious Building (no particular period)
City Square/Markets (not from any period in particular)
A castle (in any state of repair)

...
And for the Big Challenges you have some of the bigger more epic things. I know I haven't joined a Lite Challenge cause I'd bang something out and entirely forget about it before the challenge was judged...