A newer Mezannine card upgrade would be neat but one thing I think you should do is go review the recent thread where people posted pictures of mezannine hardware. In general, each individual card that exists is just about the maximum amount of additional hardware an iMac will fit.
With that in mind, unless you can radically alter how big each component is, I don't think you can fit all of those things onto a single mezannine card. a G3/G4 CPU can only get so physically small, and a SCSI controller can only get so physically small, so-on and so-forth. Plus, all those things need connectivity and if you were doing a multi-function upgrade you might need bridge or switch chips on the board, which themselves take up room.
You may be able to put, IDK, a single really really huge FPGA onto a mezannine card and do some neat things, but by the time you've added a new CPU, new graphics, a TV tuner, and SCSI to an iMac G3, you're most of the way to... just figuring out how the power circuit works and building a completely new motherboard. It would be a better computer, in the end.
So, to second what cheesestraws said - if you wanted to try your hand at this kind of project, maybe people have ideas on where to start in terms of documentation or projects you can do to get you primed for what you'd need for this one.
That said, though, if you want a G3 with scsi and a/v in and out and a faster CPU and a faster graphics card... maybe you do, really, want a PowerMac G3, a newer slot-loading iMac G3, or an iBook G3. All those computers have most of what you're asking for here.
This is kind of tangential here but one of the things that's most interesting about "limited" computers that have some but not a lot of expansion is seeing how people use the available expansion. The Mezannine slot in the early iMacs really is a great opportunity but when you're setting up a system, you absolutely have to think about what, specifically, you want a particular machine to do, and, unfortunately in this case "everything" isn't a realistic answer.
I like your optimism for the iMac G3, they're great computers in every sense... they just... aren't PowerMacs. And, the PowerMac G3, either a beige or a blue-white is really where people were going if they wanted to make no compromises at all.