I was going to just do Nurgle Daemons - my last two (my only two!) painted armies have been Nurgle Warriors,
and Skaven from the Clan Septik. Their rusty, rotten look and alliance to Pestilens made them a good fit for my WoC.
So how to mix it up a bit and make it a fun project? I remembered I'd picked up the Sylvaneth horde box last year, in a flush of enthusiasm for all things AoS, to then just assemble a Treeman before realising I needed to paint my Skaven for a tournament... and then really just do nothing but paint Skaven for several months.
My inspiration in picking up the Sylvaneth was the stories and battle plans in the Quest for Ghal Maraz, so my plan now is to build and paint both the Daemons and Tree Spirits together, converting the Daemons with all the many, many spare branches etc that the Dryads and Treemen have to recreate the corruption of nature that Nurgle is wreaking. The Sylvaneth will be painted in an Autumnal theme, verging on rot in places. Hopefully I can then field some epic themed battles later in the year!