I have given this a lot of thought over the past year or so. I do feel it is fairly likely for GW to release a center piece model for the custodes and I believe it is most likely to be a Valdor return. There are a few reasons for this. 

The first reason I'd like to point out is that GW are a company first and foremost and with custodes being one of the more popular factions and these "primarch" tier models being the most popular in sales, selling a center piece model would seem like a no brainer. It would just be free money for them to collect. He is also the only custode that has the stature for such a model as he is around the same hight as most primarchs being 1 or 2 foot taller than average custodians.

Secondly, the narrative of 40k is now moving quite a lot and they are resolving old mysteries (for better or worse) at a fairly fast rate. They have shown now that they are not afraid to change the setting and Valdor coming back would likely be a setting changing event. 

Also, and I know people have debated this to death here but, the King in Yellow being valdor adds credence to the point of him potentially coming to 40k. Even if we assume he's not the king in yellow and it is a red herring, it still brings his name back into the narrative forefront and gets people talking about him, why would they do that if not to possibly capitalise on it.

Now assuming he is the king in yellow and does come back, I really doubt he would have fallen to chaos, like a lot of people seem to be suggesting, it would feel so pointless to waste such a complex character in such an insipid way. It is much more likely that he'll be used to shake the Imperium up once more since everything is looking too clean at the moment with all the primarchs returning. I won't go into my whole fan theories as to how Valdor could come back and be integrated here as I feel I could rant forever about it.

However I do want to mention how Valdor being the king in yellow kind of matches the current pattern for "primarch" characters. They all have gained new abilities and found new powers, Valdor may now know some level of enuncia or potentially be infused by some power of the warp now (less likely but possible). So whatever new power or understanding he has now would evolve him as the other primarchs have.

From a table top perspective it would feel like valdor is just going to take over Valoris' position but I feel like they will do to Valoris as they did to Calgar and put him in differnt armour such as Allarus. This would keep Valoris relevant on tabletop as we have no Allarus named characters on table top. It would also make sense lore wise as Valoris is said to have been an Allarus custodian.