It could been scheduling, but I do wish at least one character from
any of the shows appeared. In particular, I think Kara or Jefferson should've been there. The Flash/Supergirl crossover was the first taste we got of these shows crossing universes and a glimpse of what was to come, and Kara's got a good bond with Barry.
As for Jefferson, he and Barry really got to bonding during Crisis and he filled in that mentorship role for Barry when Joe was not around. You can maybe say the Supergirl quota was already filled with Dreamer showing up earlier, but for a finale that ended not only the series, but this shared universe, I think at least one character from one of the other shows should've been there.