It'd be great to see the PPC coming back into play.
Past Winners allowing to participate
Without any kind of prize it's difficult to see why people would want to enter again, but there's no way to know without trying so it's a good idea to let them take part. Contests like the C&M Reviews contest have supporter tiers as prizes, so perhaps you could look into doing likewise with the PPC? It'd certainly spur on the non-supporter category (assuming they'll still be divided) and I'm sure the supporters category wouldn't mind an extra tier or two from taking part. The only down-side I see is that members that haven't won may feel pressured by taking part with previous winners; but we won't see if that's a problem till it happens. Also on this note, it could be interesting after a few more PPCs have run to see a PPC where only previous winners can take part - sort of a competition of 'PC's best designers'. It goes without saying that if previous winners do enter these contests again, they shouldn't be able to re-enter an old profile.
Possible Themes for the contests
I love this idea! To be honest I'm kinda surprised it wasn't put into place before haha. It's possible that people might not want to keep a themed profile for the duration of the contest, but I think having themed contests would definitely make things a lot more interesting. It'd give people an easier time deciding what profiles to make and this'd hopefully encourage people whom in the past haven't entered due to a lack of ideas to enter now since the theme has already half been decided for them. Plus it'll be a lot easier for people to vote/compare profiles due to them not differing in such extreme ways.
Timespan between each contest
I think seasonal works best. As in Summer/Spring/Winter/Autumn. 4 a year per section wouldn't be too much if each contest runs for a say a month, especially if previous winners can now enter, but even so this doesn't have to be decided right away. Perhaps base the decision of how frequent the contests will be on the activity of the next contest. If people have fair warning of when the next contests will be (eg the 1st day of every 3rd month) then they'd have time to prepare and stack up some ideas for when the time comes. Although if unfortunately the next contests aren't so active, then there's no harm in holding them only bi-annually or annually.
I'll volunteer for helping run the PT PPCs when the time comes. Whether it's hosting the contest or collecting/tallying votes, I'll be more than happy to help. I'd love to see this contest being a really active event.
I think having prizes for 2nd/3rd place would be good. I think it'd definitely encourage some people that are less confident about their designs to enter since they won't have the mentality of "There's no point entering with all these amazing profiles to go up against" if there are more prizes on offer. I know I've personally been in this frame of mind with previous contests back when I first noticed them taking place, so I really think this'd help. What those prizes are though is a different matter; I expect they'd probably end up as emblems since there isn't a whole lot else to offer (which would also make the supporter tier thing mentioned above even more sought after for 1st place contenders), but if there are alternative ideas then it'd be interesting to see something new or different as a prize than a standard set of emblems.
Also more advertising for the contest wouldn't hurt. Eg for one of the previous PPCs, the Pokemon Clubs IRC channel got wind of it and 5 or 6 of us ended up entering as a result. If this was applied to other channels too and other places like blogs etc (although I know you've already tried that), then it certainly wouldn't hurt the activity.
(By the way the colon key on my keyboard is broken so when you're reading that back, imagine it's full of smilies to make it seem more fun and exciting!)