My question would be how does this Tomb-Lizard synergize with a Tomb King army?
Reason for this: just because a slann costs a certain # of points in a Lizardman army doesn't mean he should cost the same in a Tomb Kings army. If he brings something so valuable to the Tomb Kings that could potentially make in super-OP, I would be inclined to bump up his points cost a bit. Then again, for any home-made rules I would be inclined to err on the side of over-costed versus under-costed, just to make sure things stay civil in friendly games.
This is an excellent point and something I tried to consider when creating my rules for Ra'nubis (whether or not I was successful or not is a different story, and I love any feedback in regards to it). Here are some of the considerations I made:
Magic: In terms of magic, Ra'nubis doesn't overshadow what TK special characters can already do. Arkhan the Black is a level 5 wizard and can shift 3 dispel dice to power dice. Grand Hierophant Khatep has loremaster, so the precedence is set for Ra'nubis to also have the ability. The main magical bonus Ra'nubis brings to the table is that he is better at healing via the lore attribute.
BSB: The ability to be both a BSB and Hierophant is something that TK can not normally do. However, the BSB ability is actually much less important to TK than it is to Lizardmen. Most TK lists don't even feature a BSB. So this might actually be the reverse of your point, if only to a very small degree.
Survivability: This is were your point is absolutely spot on. One of the weakness of TK is that they really need to keep their hierophant alive (to avoid army wide crumbling). Typically, TK hierophants are extremely squishy (Arkhan is a bit of an exception with T5). Ra'nubis is extremely durable in contrast to a typical TK hierophant, as his Slann status grants him increased toughness, wounds and a built in ward save (although TK hierophants can access the ward save via magic items). Additionally, Ra'nubis has the one time ability of a 2+ save against anything, the ability to heal from combat (although a limited one) and a one time ability to regain d3 wounds. So this is what might be considered more valuable to TK than to Lizardmen (losing a Slann is pretty devastating to our Lizards but not to the same extreme as TK and their hierophant).
I am very interested in hearing more about this

, seems like there is a wealth of potential material here.
Most of the stuff I have come up with is unwritten and resides purely in my mind. I've come up with it purely as a backdrop to my personal wargaming. I have a lot of material, but have only written and finalized a small portion of it. Putting all of it to page, would be a monumental task and a huge time drain (and I have to start painting some models for my new paint blog!... isn't that correct?
@Crowsfoot ). What I am interested in doing is coming up with a finalized set of rules for all of my characters and a short write up (1-2 pages) in the style you would find in a WFB army book.
This is actually one of the few things I have actually completed a write up on (many years ago... as part of my first custom character). I wrote it during the last days of 7th edition, which explains the format/layout. I've attached a copy for you if you are interested.
A corrupted slann? Also interesting; what could make him fall to the dark powers that are so anathema to the rest of the race? What just-as-planned-plan did Tzeetch create to actually pull this off???
And a rogue slann too? Who dies and gets ressurected as a minion of the undead undead-haters??????
What makes a Slann go rogue? Did he think he was doing the right thing in his own mind? Pursuing a greater good that ultimately spelt his downfall?
They actually both learn the same devastating truth (although they learn it through different means).
Tzeentch uses this to twist the mind and ambitions of Vokul'nax towards the path of Chaos. Vokul'nax is attempting to manipulate Tzeentch and Tzeentch is attempting to manipulate Vokul'nax. Such a game of deception would clearly interest Tzeentch. Either way, Vokul'nax comes to believe (either rightly or wrongly) that there is a pathway that he can follow to achieve godhood.
Ra'nubis on the other hand pieces together clues to uncover the same truth as Vokul'nax did (with the exception of the pathway to godhood), but he is armed with one more bit of information and instead continues to follow the trail of bread crumbs.
So it is the same stimuli that sends them on their very divergent paths. Vokul'nax becomes obsessed with power, while Ra'nubis becomes obsessed with uncovering truth. These pathways are a direct result of their own individual personality traits.
I have all but one spell finished.