Proposal: Help sell the science with examples of several plausible scenarios. Here are some possibilities:
1] A tentative DM signal is reported by LZ/Xenon1T at the low-mass end of their sensitivity region (~ 1 GeV). CMB-S4 measurements could potentially see this signal, helping confirm it, and helping understand the physics behind it. A new-physics signal will need to come consistently from multiple data sets, and CMB-S4 could be a part of the arsenal aligned at understanding the nature of dark matter and its connections to physics beyond the Standard Model. (Note: i) This statement depends on the interaction model; nuclear-recoil-based direct-detection is typically much more constraining than the CMB for masses > few GeV, but can't go below that. ii) To compare constraints, we need to map between the cross sections shown in direct-detection exclusion curves and the momentum-transfer cross sections talked about in cosmology literature (Vera and collaborators worked this out for effective DM theory). iii) There are other experiments that might be able to probe this region of parameter space too...should investigate this more.)
2] Tension in H_0 from SN and CMB persists, with N_eff the simplest way out. CMB-S4 and LSST/WFIRST SN confirm this tension at ~20-sigma level AND N_eff resolves it. Short baseline neutrino program finds evidence for sterile neutrinos with mixing angle so large that in the standard oscillation scenario, N_eff should be 4, not 3.5. This could point to a large lepton asymmetry ro some another non-standard production/suppression method in the early universe.