I posted my thoughts on reddit so I'll just copy what I said and show you my take on how to improve his kit.
Landslide. Currently it needs no changes. If we remove the stat stick feature, then all warframes require another ability rework that depend on stat-sticks by adding a modding feature and only show those mods that benefited before. Oh, and you'll need more forma if you want their abilities to be as effective as it was with stat-sticks.
Tectonics. Ok, this ability is somewhat useful. It can block doorways, cover defense targets and cover yourself and allies if you're in a pickle. But its boulder part is underwhelming. Even if you boost it with Petrify, it still doesn't do enough damage other than pushing enemies away. So how to improve it? Add Gara's 4th ability scaling. If an enemy gets hit by the boulder, his health is then converted and added as total damage for boulder's explosion. Example: 3 enemies, 3000 health each, they get hit by the boulder, boulder explodes and deals 9000 damage. And, to balance things out, make it deal blast damage. Petrify boost will double the damage. (It may sound overpowered but if you're a game developer, you have to go crazy first, then balance things out later.)
Petrify. Make Atlas' passive armor decay slower.
Rumblers. Two things need to be improved. How long the rumblers can stay and their use in high-level combat. If you use Petrify on rumblers, their duration is reset. If they take damage, they deal more damage (kinda like Chroma's Vex Armor's Rage buff). And add a small chance for them to petrify enemies by punching them or throwing boulders. Also, add a tap&hold function. Tapping the 4th ability button on an enemy will make the rumblers target that specific enemy. Holding the 4th ability button will make the rumblers explode. If you use the augment, tapping the button will target a specific enemy, holding down the button will make the rumbler slam the ground and knockdown enemies.