I think merging some add-ons into the Xymon source code should be considered where those add-ons would be widely used, subject to licensing restrictions.
For example, and I have not yet tested this add-on, but a way to alert on processes using too much memory looks increasingly useful to me (together with graphing of memory for certain processes): http://tools.rebel-it.com.au/xymon-procmem/ Ideally the shell code that add-on uses would be converted to support in the C code so that this could be configured in analysis.cfg rather than hosts.cfg though.
Another example is Xymondash: https://github.com/daduke/Xymondash I haven't used it yet either, but it sounds good. Having said that, development on that shouldn't be stymied by locking it to Xymon releases - and it requires Python >= 3.5 (or 3.4). Maybe as a post-installation task Xymon could ask if you wish to install it, check dependencies, ask a few questions, download and install it? It would be good to be able to present a more modern GUI as part of Xymon.
Kind regards,
SebA