Just wiring two pins together

An additional comment regarding I2C repeaters.

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If you look on the Linear Technology web site, you'll see lots of I2C repeaters and re-drivers. These parts are essentially intelligent devices that look at the logic levels. To create an FPGA I2C bridge, you'll basically have to do the same thing.

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There are more decicated repeaters available from NXP. They all depend on special interface hardware, in so far they can't be implemented in a FPGA without similar hardware that either senses different logic levels and/or pin currents.

As Dave explained, the only way would be to implement an I2C bridge that decodes the bus protocol. It's behaviour would be partly different from a "wire".