A few Windows games have problems with too high a clock (> 2ghz, for instance), so forward compatibility isn't always something you can count on.

Like others said, it's a pain to get "One to rule them all" working (to bypass software and hardware limits - depeding on how much $$ you put and what is available to buy for you).
Here's my spin on it (using i7 980X) : LINK
And here's ruthan's (LGA 1366 Xeon) : LINK

You can downclock most Core 2 stuff (and later CPUs), around 1GHz mark (1,2GHz - 600MHz, depends on platform).
There are also performance degrading programs (like SetMul) that switch off L2/L1 caches if games are really stubborn about performance.

Running NVMe on old platform isn't that hard (you simply need EFI boot to get it to work).
3-rd party programs like Clover or DUET can do it with anything supporting 64-bit and PCI-e (simply put them on CF card with IDE adapter, and you are set).
NVMe rules for old platforms, not because it's fast, but because it bypasses SATA2/SATA1 controller (and any of it's shortcomings).

I could do build without ISA by using Vortex 2 with Dreamblaster X2. You could throw in OPL2LPT or OPL3LPT for good measure (if your board has LPT port, if not think about PCI-e expansion card).

Biggest issue with "one to rule them all" however, is HOW MUCH compatible you want it to be.
If you plan on using it as daily driver, my and ruthan's PCs are best (mostly WinXP+ and occasional native DOS). BUT if you plan on playing really problematic games, Super Socket 7 or Pentium III/Slot-X platforms may be the best.