My biggest concern is that the infantry probably would have trouble fitting onto 20x20mm square bases. I did manage to get Fyreslayer Vulkites to fit quite well, but unlike them the Bonereaper infantry do have particularly wide shoulders like Necrons do, so that could cause problems.
However, I refuse to fall into the trap of searching for Tomb Kings stuff on EBay, as that’s what the traders like to prey on - desperate players so eager to get something from the old Tomb Kings line that they’ll pay anything.
I still maintain that the cavalry could work as Necropolis Knights though, considering that Necropolis Knights move at the same speed as a barded warhorse and that those new cavalry would certainly fit on the chariot bases used for Necropolis Knights. Toxin sacs could be attached to the horses in the style of Tyranid models to represent the Necroserpents’ Poisoned Attacks, and a spear option can be seen clearly on one of the models. The riders also don’t seem to have the joker faces that the infantry have, and a combination of the right paint scheme and perhaps some looting of old Tomb King bitz on EBay (which are generally cheaper than finished models) would sort out the aesthetic issues. Also the priest model would be great on the Casket of Souls as he is in the act of releasing or ensnaring souls for his use.
Generally though I think I will revert to waiting to see what Mantic do with their Empire of Dust range. They revealed some time ago that Empire of Dust would feature in the main rulebook for their upcoming Third Edition, meaning they should be getting more support than they were when they were first devised in Second Edition’s ‘Uncharted Empires’ supplement in retaliation to GW killing off Tomb Kings. This honour should result in more concepts and models being developed for them, and these chaps certainly look more suitable for Tomb Kings (I’m showing you this video because the paint job on the website doesn’t do them as much justice as it should):
I like how these are a lot more adorned than the old GW ones as well as being less bulbous around the head and torso. They are also priced more like the GW of old, with 20 Skeletons for £20 - much more affordable than most stuff on EBay and almost certainly cheaper than what these new undead are going to cost. The only disadvantage with Mantic is that they are so incredibly slow at developing new stuff that they make even the old GW with their monthly releases look fast.