Possible MSX Kickstart Project ?

Overcloking a Z84C0020VEC is one possible option...I believe these can be Over-Clocked to 28MHz+ with no ill effects... then use a Z180 as a second CPU. This would allow all existing software to run and future software to take advantage of the Z180's extra features and speed.

Using an FPGA is trendy at the moments... but if your going to go down that route why not just buy an ALTERA DE board ? I think most MSX users want something that is basically a Faster and Better MSX2+ , but still using Retro (old School) components, Z80 CPU, V9958, SRAM, EPROM etc with perhaps Z180 second CPU, a second VDP, better sound PSG etc ?

You thought wrong. Most MSX users doesn't care about a new processor, they already have the R800 since 1990 and not a single one of them cared to use its features (but its faster speed). Maybe a lot of little tests and proof of concepts, but I can't think of any R800 specific code. And if we didn't cared about R800, I don't think any of us want to learn to code in a another cpu, even if it's similar the z80. Sad but true.

make dual cpu system one z80 one other z80 like

Oh why not say, a 16-core Z80 system? (probably easiest in a big FPGA, but also doable with discrete Z80's). Because: such a beast would be very difficult to program effectively. So close-to-0 software would be written for it. Read: it would be much fun / interesting to design and build. Very nice as proof of concept. And very, very cool. But with near-0 practical use.

Besides: why go multi-Z80 route when an FPGA or other Z80-like cpu can do 100 MHz+ equivalent Z80 speed? Elsewhere, cpu's went multi-core because clock speeds couldn't (effectively) be boosted further. For our humble Z80 that's a non-issue.

Have you ever used R800 specific instructions for anything serious?