I wouldn't be surprised if Fenix is added if they do have him have Sentinels too but possibly with some new ability.
I think the units chosen for Karax all make sense. Even giving the Khaydarin Monolith makes sense as there is a comment in the campaign when you first get it about how quickly he adapted it for their use and as you say barring bringing back the dead there's no other Nerazim commanders they can use (kinda sad really that there are so few named Nerazim now...)
The difficulty for Fenix is how to make him different from Artanis? Based on his abilities in the campaign and his personality he seems like he'd be based around barreling into the enemy, a brawler if you will. Though it raises a fun mental image if he were to get the Summon Fenix ability... "I summon... ME!"
An interesting thing I could see revived for a commander is in the Void campaign files there are upgrades that suggest the Immortal at one point would have had a self ressurection ability (like the Sentinel or Thor in WoL)
I do wonder if they might bring back Urun who got sadly forgotten in the campaign as a fleet commander? Selendis might fit for that role too though there are potentially lore issues with her, depending on how much they worry about the rough timeframe Co-op missions have (the Shakuras Temple mission already standing out)
I also think a ranged-focusd Protoss army could be interesting so Adepts but no Zealots (perhaps this faction would have some of the requirements/cost of Adepts reduced to balance) simply to mix things up a bit :)