What should be added? No new classes.
What I think will be added? No new classes.
What I'd fine with if they did? Any of them.
Artificer is an Eberron class for the Eberron setting and thus I don't feel needs to be in the new PHB (especially after being re-printed in Tasha's.) The Psion I do not believe would get enough space and page count in the new PHB to make people happy with it (especially if they want the Psion to not use spells and instead want a whole new system for it). So it should be saved for the Dark Sun campaign setting if/when that happens (as it'll be able to get more page count attributed to it.)
Summoner and Swordmage are two classes in my personal opinion that do not have a specific story, flavor or "job" for their place in the game world, which is why we haven't seen them stick around from times past and I don't think we will see them going forward. Other than the Core Four (which can afford to be more generic)... any other class added to the game appears to me to need a story reason to exist in the PHB. And something like a Summoner-- a class that can summon all manner of different types of creatures-- is more a mechanic to be used by existing classes than needing to be a class on its own. And as far as the Swordmage, the one consistent story of the class I always seemed to see has been "Arcane half-caster", which to me isn't a story, it's an empty space in a grid. And thus is no better than the Eldritch Knight and thus doesn't have a reason to exist in my own opinion. But if they were to get made and appear in the new PHB it's no skin off my nose.
And Warlord? It's fine. It's a fine class idea. Was fine in 4E to fill out the grid, and it's fine now. If they add it, okay, if they don't, won't bother me.