Pete, in thermodynamics, you can plot out a process (say the 4 step cycle of a 4 stroke engine) on a graph. The area under that graph is known as the hysterisis loop, and shows you how much energy you've lost performing the process. The same idea can be taken into anything that involves energy (hysterisis loops are pretty popular in electrical engineering) - say the transmission of power from foot to fin. The main aim of efficient fin design would be to minimise this loop.
Your idea of foot pockets would do exactly this - aim to achieve a minimum energy loss. There's other techniques as well - a pretty popular one is the use of rails to prevent vortex transmission and reduce drag. I've never seen a mono with a big rail, but this is something you might want to consider in your design.
As for placing the blade inline with the body, I totally agree - I've drawn a few mono/bi fin sketches in the last few days and every one is inline. The only blades I can think I've seen do this off the top of my head are C4's.
Ideally, you want to get maximum thrust with minimum energy expenditure. I crunched a few ideas through the equations, and the way I see it, the ideal fin shape is entirely dependant on the swimmers stroke, and their fitness level; The fitness of the swimmer will give the allowable resistance, and their stroke will give the ideal bend to minimise the hysterisis loop.
Anyways, that's a bit of food for thought - let us know how your fin making goes
