Bad: Apparently, Dax met not only McCoy, but Christopher Pike, Sarek's dad, the Parasites, and was also working on the Excelsior. For a random character from a reclusive world, this is just a little hard to believe.
I think the keys to keeping such crossovers believable are:
1) Don't overdo it. If you have your characters meeting up with familiar characters or elements from past Trek history, include enough original characters or elements that the whole thing isn't just one big reunion. For instance,
Immortal Coil would've been more plausible if only some of the collected AIs had been familiar rather than virtually all of them. In
Orion's Hounds, I brought back two familiar spacegoing life forms, the "Farpoint" star-jellies and the Crystalline Entity, but I avoided having the entire cosmozoan biosphere made up of critters that one
Enterprise or another had happened to run into.
2) Have a reason for it. Don't just drop in a familiar character or element for the sake of a cameo, and make it plausible that the character would be involved in the story. In
The Buried Age, I initially wanted Worf to play a larger role in the novel, being involved on the mission in Part IV rather than just showing up briefly at the end, but Marco thought that was overkill given the other TNG characters I was including.
3) Don't interpret canon selectively. A story establishing a family relationship between V'Ger and the Borg doesn't make sense because the differences between the two outweigh the similarities and it's contradictory that a purely cybernetic entity that doesn't even know what humanoids
are would be spawned from a cyborg race that assimilates humanoids the way Morn downs drinks. And Trelane as a Q doesn't make sense because he was nowhere near as powerful and needed a machine to do his tricks. Drawing a connection based on one similarity between two things is a bad idea if it requires ignoring canonically established differences or inconsistencies between them.
Just putting it out there.)
