Accordance contains the works on Divinity by John Gill, but not his commentaries. I think that would make a nice addition.
Also, we have Gesenius' classic Hebrew Grammar, but not his Lexicon.
And while we have the Expositor's Bible Commentary, the older Expositor's Bible is not available as an add-on.
My last suggestion is the that the texts of the Unlocked Literal Bible and the Unlocked Dynamic Bible be considered as an add-on... www.unfoldingword.org/bible
Thanks for a great set of resources and for the amazing level of support you have provided over the years. Accordance is a remarkable tool.