I think you have a lot of great ideas here! I was wondering the other day if the lack of dice in the melee phase would kind of limit unit diversity, but your Tomb Guard rule seems like a cool way to add some flavor to it. I also like your "Plentiful" rule, and I imagine we will be some variation of it as the game is expanded.
I like your "Protected" rule but I think I'd make it short range. Medium range actually covers a lot of battlefield and it would be hard to picture a unit at full medium range sprinting over to take an arrow for the Priest. Again, I think we'll be seeing something like this as the game game grows.
My personal favorite is your "Underneath" rule, and I especially like the chance that it can kill the targeted unit outright. The image of a giant scorpion bursting from the earth to pull some hapless unit to their doom is just too cool
I hope FFG takes note of your rules and considers making a Tomb Kings army in the future. But, as I'm sure you already know, Tomb Kings are pretty low on the totem pole in terms of popularity. You have just as much chance as getting your Tomb Kings as I of getting my Beastmen I'm afraid.
Elite Disk, Construct
at the start of combat roll a d6 on a 3+ deal 2 damage to either a disk pinning this disk or to a disk pinned by this disk. Repeat this ability until you fail to roll a 3+.
Attack Str 0
Counter Str 0
Elite Disk, Undead
Ranged attack: Siege, 3 dice, 3 damage
if you roll a hit (even when at siege range) after the attack is resolved you may move the targeted living disk 3 directly towards its deployment zone (if their are multiple zones you chose one).
Either raise the toughness of target undead disk by 2 until the end of turn (the effects of this spell last until damage tokens have been removed) or bring a skeleton warriors disk into play with in long of a caster. This disk is removed from play during the end phase.
The King Commands
in addition to activating the two units from this card you may activate a Tomb Priest. Do not put an activation token on the Tomb Priest.
Idea behind The Tomb Kings
Pretty much they're a team that is weak while they're pinned and strong when they're pinning. They also rely heavily on their Tomb Priests. Because they're so slow they really need their priests to get them moving and be able to pin enemy disks. Their disks are also most likely going to be cheaper in points so you should be able to have more disks then your opponent which will allow you to sacrifice some weak cheap disk in order to get your good disks into a position where they will be able to get big damage. They have some pretty good ranged options too and their chariots are pretty versatile except for being junk when they get pinned. They have some whackier random chance stuff too with the Tomb Scorpion and Bone Giant.
Let me know what you think about my ideas for the TK.