Atalante seems like a sure-fire "in". Especially after the two leaked images surfaced. It's why the list is so believable.
Welfare - Known
Takeuchi unit - Known
Another "big" unit - Known
Related to the male 5-star - Unknown but highly likely to be Atalante
Three slots remaining. One is Wada Arco, so you just pick from the small list of available units there. The main 4 are Drake, Elizabeth, Passionlip, and King Protea. Drake has a pre-event banner now, so she's likely out. Elizabeth is only associated with Halloween by rubes, so she's out if trying to make a convincing list. So you can just toss a 50/50 on the remaining two and it'd be believable.
The other two are just random slots so all you have to do is not toss SUPER popular units there, since if you do people would just call it a fake fan list right from the outlet. Make some unpopular choices and people are more likely to believe it. Anastasia is an easy slot here, because Kadoc's VA is in the stream. So just toss one unpopular Servant and it's a believable list.