Assuming you start on teh two sticky out bits, bowl is the best. IMO. Then teh center Rock one, but the other 3 we all have in one form or another, while bowl is more original.
BTW, If you do the last one, any chance it could be like a quary? IE grass on top and then huge cut out rock for the middel... that would feel so cool to play in.
Bowl would be great because, if the center were metal rich, you would have to decide between advancing on the sides for a tactical advantage or advancing in the center for more resources and a larger army.
Personlky I thik we have quite enough "rush the middel for metal maps" so whatever you do, be carfull with the metal. The free land there should be the hook, not vast amounts of metal.
dry river and center rocks. the others are good but its too focused on the center. i like games where there is not particular area of occupation that wins you the game but more so maps that you have to play well to win. you still have to play well even if you own a metal rich position to hold it and win, but if maps are more evened out, players are forced to expand anywhere and take up a certain line.
I'd love to see any of those done, they look like some absolutly great 1v1/2v2 concepts and I think spring could use some more of those. I like how all of them use map heights for tactical purpouses a whole lot, not enough spring maps do that and the ones that do have tended to go a bit over the edge.
Talk to forboding angel about balancing maps with height differentials, there's a few tricks he's figured out that could make you job easier.
I think if you do one like center rock and widen it out a bit. (make a bit more low ground so that it doesn't play like "Two Valleys") I think that would play really well as a map for epic battles, (players could put lots of Arty on the top of the center rock and pwn all who oppose in the valleys!
I like the assymetry of dry river; but bowl is the most innovative.
Personally, I see no problem with having the middle as metal rich, because the design of the map doesn't necessarily make it a "king of the hill/middle" rush to the centre. Because of the low terrain in the middle, you have a situation where instead of the middle being a strategically important place to hold (like most rush to centre maps), but rather a particularly dangerous area to hold. The outside rim would be far easier to hold; and a far better route to your opponent's base; the centre, on the other hand, would be in big trouble if you got some medium artillery (guardians, etc) up on your base; because you could probably shell much of the bowl.