This list also won't have access to Ordinatus or Forge Knights presumably? That'd be other disadvantages / lack compared to AMTL.
I'd prefer Titan Tails to be an upgrade available to any Chaos Titan personally; they're often depicted on any sort of Chaos Titan and not just in the Titans of Khorne. It seems rather appropriate for a Slaaneshi Titan to have a tail between it's legs

Are there any AMTL weapons the background says were only invented after the Heresy and could be taken out from this list? And /or how about limiting some weapons which might feasibly be harder to maintain? - thinking possibly of some/all of the plasma based weapons. Also maybe remove the option to have one-shot titan missiles? Maybe the Dark Mechanicus would'nt have plentiful stocks of these.
I quite like the idea of having worse base initiative for the power's titans, but then giving them bonuses to certain actions appropriate to their power. This shouldn't  be hard to remember if you're only having a single power's titans in the list.
Some off the top of my head ideas and brainstorming thoughts (not necessarily balanced or a good idea, just throwing ideas around):
Could Tzeentch Titans have a
Flames of Tzeentch upgrade (or whatever the Tzeentch only vehicle upgrade from W40k is/was called - I can't check because I'm not home) where the titan is covered with flames or mouths/whatevers emitting flames. Game terms something like +25ish points +1 CC attack with first strike.
Following this pattern for other powers Slaanesh could get some kind of Lure of Slaanesh upgrade, with alluring scents / disruptively loud noises distracting and affecting nearby a nearby unit / all units within 15cm and making them -1 to their initiative roll.
Nurgle could get Plague Wind / Miasma of Nurgle / Cloud of Flies giving a chance to put a blast marker on a nearby unit.
Khorne could possibly get an upgrade that made them even better in close combat, but if the titan fails it's action test it
must charge and assault the nearest unit, friend or foe.