Well, every system is a product of its restrictions and what was ongoing at the time of planning.
Without anything to work from, pretty much any option is open.
MS possibly could have mirrored the CPU die so that there would be 6 cores on-chip with two L2 caches shared between each triad. It would be larger than Cell, and more costly, but overall it would probably be easier to utilize and have much higher performance for non-multimedia tasks.
They could have gone hog wild and went for more RAM, possibly some dedicated to CPU and GPU, and a hard disk.
If the gaming division didn't care if HD-DVD became a niche product, they'd have an HD-DVD, and probably suffer limited production quantities.
The cooling system would need to be revamped and would be much pricier and louder, but hey, no constraints.
The hardware would likely sell at a loss for most, if not all of its lifetime.
Then again, without any design constraints at all, why not just buy a gaming rig from Alienware and put an X360 sticker on the front?
The constraint is the similar pricing to the ps3 but with more powerful specs. I don't think M$ would have launched a weaker console along side the ps3 for the same price range. This is a hypothetical look at what the console could have been had it launched with the ps3 last month.