Once T212843 is deployed, we can start enabling Lexeme access to a first set of projects for test reasons.
An obvious candidate is the beta cluster for the first deployment.
I see that fr.source has already a request pending in T203220
Given the descriptions of the use cases on fr.wikisource (linking individual words in texts) (the use cases, by the way, are enormously cool!) it is possible that we would very quickly have a lot of usage, and so I suggest we do not use it in the very first wave.
I know that some community members from other objects have expressed interest - but they often seem to come from large Wiktionaries (eg. ru.wikt, zh.wikt). I would recommend not to start with the large projects yet, because if something goes wrong it is more likely to cause issues due to size.
I am suggesting (entirely biased, obviously) as a community member to use hr.wikt as an early production project to try it out (so far no one stated opposition, and as you can see in the recent changes of hr.wikt, it is likely no one will, given the low activity level on the project): https://hr.wiktionary.org/wiki/Wje%C4%8Dnik:Konoba#Bilo_tko_protiv_eksperimentalna_upotrebe_Wikidate_za_leksikografske_podatke?
Another interesting candidate would be eu.wikt - maybe @Theklan would be interested to see if there is opposition.
Further thoughts?
Given the timing of the upcoming WikidataCon, it might be interesting to have this in place before the WikidataCon.