Most of the long-horizon reports that I read centered around Spectre 1 (intra-process OOB reads by racing bounds check against conditional branch prediction, currently mitigated by lfence), which is still a fundamental-principles issue that would be exceptionally challenging to solve without either substantial performance loss or some form of userland/compilation-level workaround. Intel still haven't indicated a hardware fix for that one. I expect that most mitigations for this will center around improving static analysis tooling and giving the compiler finer-grained control around fencing to reduce performance impact.

Variant 2 (branch target injection, retpoline style software mitigation) is the most interesting. There are certainly obvious theoretical hardware mitigation strategies like tagged BTB entries and this is the most interesting hardware fix. To me it's very cool but not particularly surprising that Intel believe they'll have pure-hardware mitigations for that soon.

And Meltdown was frankly an Intel mistake that would have been shocking if left unfixed in the next hardware revision.