The most promising way to improve the accuracy of the methane measurement is to remove the CO2 before using the TLS instrument because that would concentrate the remaining gasses. However, it will take several months before this will be done because of other priorities. One reporter asked about the priority given the wide public interest in the methane question. Apparently though the highest priority now is the use of SAM on soil.
I would think the methane work has a higher potential for immediate discovery than the ingestion of the dry sandy soil in the current location.
Other future atmospheric measurements will include H2O and its isotope rations.
By the way, the average pressure measured by the REMS instrument is now above 8 millibars. (Also it has been pointed out by a UMSF poster that 8 millibars at low Mars temperatures is actually a denser atmosphere than 8 mB would be at typical Earth temperatures.)