ElDude and King4aDay for starters.
This isn't about dumbing down the competition but making it more appealing for a broader spectrum of photographers and generating more interest and more submissions. In fact dropping themes or extending the submission time will almost certainly improve the quality of submissions and thus make the competition even more interesting.
You're trying to give the impression that there was a great untapped source of photographers active on the forum who would not enter the competition due to it having a theme. Sure some struggled with the theme (I did every month!) but participation was still decent, as already mooted perhaps the themes should all be thought out before the comp starts. You already listed a number of approaches that you used to help you with photography, guess what they are also themes that could be used to help everybody.
Removing themes and extending the submission time isn't likely to improve the quality at all. Nobody will be shooting with the competition in mind, all anyone will do is pick their favourite shot just before the deadline from the previous months. Currently we already see those in the likes of the PYPH thread (or did until it was closed). All you'll be doing is adding a scoring element... It's not going to drive creativity or discussion. Even with the themed competition, you'd at least get discussion over how much everyone hated the theme
I don't know what the answer in this forum is, we've tried pic threads with a topic, we've tried critique threads, we've tried PP before and after threads, we've pushed the "post your own thread for critique" etc. type threads. Sadly the only busy threads tend to be the ones slinging matches over gear
In years gone by we have the likes of Mr Jones, famas, Gaffer, Martin Turner, SteveOBHave, Derek W, Hodders, Willis, robertgilbert86, Wedge, ScarySquirrel, Greedy123, Oxygen, p0ss3s3d, Mud, PaulStat, sketch145 and others (even TBL!) who used to be active in the forum discussions and also sometimes the competitions. All of them posting varied work, in addition to those still around today. Traffic and discussion used to be much higher. Perhaps we just don't have the right type of people at the moment to be keeping the forum alive and buzzing? I know I'm certainly more of a lurker than a poster!
The only regular discussion and banter we get now is arguing over gear! Perhaps a gear sub forum, and a ban on gear debates in the main photo forum might stimulate more productive photography discussion? I'm not convinced though!