I don't think a 'celebrity' works for a small character part like Father Fogden (by the way broughps - like your typo - Flogden!).
I think WS guesting in Outlander was inevitable, that he would play a quirky OTT character fits him.

Dr. Stern is with them when the rescue Ian, isn't he?
I think he's just messing with the fandom and he's not really doing Outlander. He's probably starting to film this show of him something with "better later than never" ? He just wants to be a troll as usual.
Hey!! That was in the 60s. And look now, he's won two Emmys, nominated for 7 times.. won a Golden Globe, nominated for 2 times. Quite an accomplishment, I'd say.
In terms of guest casting, Simon C. probably seemed such in Outlander, if you are from the UK and familiar w/ his work.I think WS guesting in Outlander was inevitable, that he would play a quirky OTT character fits him.
Point to be noted: WS didn't read any books to know about Young Ian's kidnap.
WS is Canadian (not sure if he has duel citizenship or not) so shouldn't have a problem working on Outlander.
Do you think he's going to end up being Amaranthus' father?
He's too old to play Dr Stern anyway.
And Amaranthus isn't a little girl.
Besides, I would never get angry trying to defend someone I barely know.
Apparently Outlander set is just down the road for WS.
Do you think he's going to end up being Amaranthus' father?
Well, this should clear things up. William Shatner isn't there for Outlander filming. He's there to join the cast of a new tv show, The Indian Detective.