Maybe a stone Howdah...

I dunno. I am providing the concept, and leaving it to others to set the capability numbers and then assign a points cost.
I am putting you in charge of this research.
Let's get started then
Ancient Bastilodon
This would be an officially sanctioned conversion from the standard Bastilodon kit.
- It would be slower than the standard Basti (...because it is Old).
- Its armor scales would be double-thick (way better Armor Save, Resistance to Rend, Negation of Mortal Wounds, and such-like). (...because the scales thicken with Age.)
- It’s use would be to accompany Saurus Infantry.
- In combat it will always turn tail, but instead of running, it starts smashing stuff with its humongous clubbed tail (...which is bigger now that it is Old).
- Damage from the tail is comparable to clubs wielded by giants.
- There is a howdah (needs to be scratch built) filled with Skink shooters. They can freely fire into the melee below. (Tall Howdah rule, we’ll call it.)
Right then let's make something utterly ridiculous an ancient that doesn't even begin to care about the enemies opposing it cuz óf how massive and ancient it is.
Wounds: 15
Move: 7", slightly faster than a normal bastiladon. A normal bastiladon is already 5" slower simply won't work. It's slightly higher cuz of the following special rule:
Tectonic pace:: the ancient bastiladon is such an ancient creature it cares not for the pace of younger beings and will move as it pleases. It cannot run or charge. It's movement can not be modified in any way. Retreating counts as a normal move for it. Furthermore it can move through enemy units provided it ends 3" away from them. Whenever it does so it also does 1 attack with its bludgeoning tail as it simply waltzes through without a care in the world.
Save: 2+.
Ancient scales: a bastiladon's save cannot be modified by rend. Whenever it suffers a mortal wound roll a dice, on a 4+ it is ignored. At the start of you hero phase heal D3 wounds.
Elevated position: the skinks sitting on the howdah are in such an elevated position they are essentially sitting on a small mountain. Their ranged weapons can still be fired in melee.
Special equipment:
pick one of the following:
The AoE option:
Shrine of Sotek:
- Roll a dice for any enemy model in 5". If the model has 1 wounds or less it suffers a mortal wound on a 6+ as it's bitten by poisons snakes. If it has 2-4 wounds it suffers a mortal wound on a 5+ if it has 5+ wounds it suffers a mortal wound on a 4+
- All seraphon within 3" are blessed with the Venom of Sotek. Double the damage of their attacks on a wound rol of 6+
The single target option
Solar shrine:
- Blessing of the sun Nearby seraphon are invigurated by the sun. Add + 1 to all wound rolls for seraphon within 5"
- Greater Solar engine; ranged weapon
The defensive option:
Priests of the seraphon
- Several skink priests ride along on the howdah in addition to the normal crew. It can now cast the celestial rite and has the priestly trappings ability.
- Starbolts: ranged weapon
Now for the attacks:
Ranged:
- Greater Solar engine: 24" 3d6/4+/4+/-1/2 deals 3 damage to chaos daemons.
- Starbolts: 18" 4/3+/3+/-1/1
- Skink Javelins: 16" 8/4+/4+/-/1
Melee:
- Bludgeoning tail: 2" 3/4+/4+/-2/3. On a 6+ to hit it deals 3 mortal wounds in addition to any of it's regular damage.
Point cost: 400-500-ish seem fair? Classified as super-behemoth. Limited to 1 model in a 1000 point game, or 2 in 2000 and 2500 point games..
Seems like it should be fun and make for a nice centerpiece of your army