Assuming you mean fiber as in fiber-optic, you will do nothing short of creating a mess on the board.
Three major problems with putting fiber on a PC circuit board (biscuit).
1. Fiber alone doesn't do anything. You need a signal repeater and terminators to handle the data traveling on the line. You can see how this will become a mess on a biscuit with thousands of fiber lines running.
2. Speed. Fiber is great stuff for long-distance, true, but in the short distances of a PC biscuit, you're actually hampering yourself with fiber. Since fiber doesn't translate directly from the chips and connectors, you still need to handle those signals and put them on the fiber lines. With plain metal circuits, the speeds are much better. Though metal lines at distance aren't as good as fiber, within short distances they are really good. The transfer from components can be direct, without a fiber interpretor.
3. Size. A plain metal circuit line requires little to no protection to operate smoothly in a biscuit. Fiber is different, though. Electrical noise is almost non-existant in fiber, but heat and light are major factors to be considered. You'd have to insulate each line, which would mean you couldn't put the routes as close together, which would be a bit of a mess.
As for heat, heat from the mobo is negligable at best. The trace routes themselves are not terminating signals or charges, and all the really fast, constant traffic, happens in the major chips and components. Your proc is actually spinning like a top while your mobo routes are waiting for the lights to change.
As for memory, it's not a matter of speed or space. There are constraints on the maximum addressable size of memory that a computer can effectively access.
Also, as long as we rely on motorized hard-drives, CD-ROMs, and buffered components, there will always be a boot-up delay. Drives have to come to stable speed, no matter how fast the system can handle the data once it gets it.
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