Their IR is a mess. So a "new LLVM" ought to start by nailing down the IR.

And as a bonus, seems to me a nailed down IR actually is that portable assembly language the C people keep telling us is what they wanted. Most of them don't actually want that and won't thank you - but if even 1% of the "I need a portable assembler" crowd actually did want a portable assembler they're a large volume of customers from day one.

Having tried writing plugins for both, I very much prefer GCC's codebase. You have to adapt to its quirks, but at least it won't pull the rug from under your feet gratuitously. There's a reason every major project ends up embedding a years-old copy of LLVM rather than just using the system version.

If you're ignoring the API and writing IR directly there are advantages to LLVM though.