I would think it has to be in at launch. Vanguard started off very badly (from a bug point of view) and from peoples initial reaction to the game, they didn't last much longer then a month or 2. If Pantheon starts off in a way where things are not implemented, then it could have the same reaction. People won't buy the game / sub to it if it isn't complete. And then once it is complete they may feel like it is too late to start playing a few months after launch when things are implemented. How many people read negative reviews about VG and never tried it even after the bugs were sorted out and the game was stable. I myself played at the start and after 2 months I had, had enough. I popped in every now and then with my all access account but never really played it after that as there was not a very big playing population left.
I don't think it is a case of get the game out in a hurry even if half baked. Seen that before with plenty of games. Take the time to finish it. Doesn't need to be totally polished but it needs to have all the components of a fully running MMO.
Doesn't need things like mounts or flying they can be expansions later on. People's feet can do the walking there, but I do think it needs to have crafting in at the starts. Not all recipes al long as recipes are kept up with the population level. (As long a the game has a slower progression than most. I know 2 months was enough in VG to max level and crafting that also helped me to stop playing since I had nothing much else to do.)