I recently moved from a pocket Moleskine as a capture device to a 5.5"x8.5" paper planner (to capture more notes, keep reference material, and do some actual planning). I'm using a selection of D*I*Y Planner pages, which are working well so far.
When I was browsing through planner ideas and trying to work out how I wanted to set up my planner, I came across an idea that Time/system is using that template designers might want to consider as a possibility.
They have some forms that are designed to fold out of the planner. To explain: think of an 8.5"x11" page with a 2-up form printed on it, with holes punched in one end. Fold the page over, leaving some room around the holes to let the folded page stick out about as much as a tab when you put it in your planner. (Of course, the same concept works for A4 pages as 2-up A5s.)
Then you have a page that can fold out beyond the regular 5.5" width when you want it -- perhaps handy for reference or planning information. I've never used the Time/system planner, but it looks like their weekly and/or monthly planning pages can be had in this size/arrangement. When you're done with the period, you can archive it by folding the plan around it, which could be handy later on.
(See the first image on this page for an idea of what I'm talking about.)
Anyway, it's just an idea. Print out a 2-up of something, punch and fold it, and see what ideas might strike you. I'm keeping a few projects together this way, folding an outside letter-size page around a group of D*I*Y Notes pages.