Let me say this. I don't like the 2/3rds proposal, though I wouldn't complain about it if it was deemed necessary.
However. I feel that Epic:Armageddon is sufficiently detailed system that it's possible to arrange a lot more subtle restrictions than that.
For instance, with few exceptions, lists with lots of one type of unit do not fare well in Epic:A. Contrast that with Warhammer 40,000, where usually the use you'll get out of one unit increases exponentially the more of them you can cram into your list. That's because the game has a lot more dimensions to it's tactics, and one unit can't typically do everything.
Titans, on the whole, are the closest to breaking that principle, and for a good reason. Despite that, I think there are enough possibilities to introduce - or expand upon some existing ones - holes in their capabilities, such as would lead to a choice of an AMTL player to either go with a full Titan army, while receiving some - perhaps slight - disadvantage but still having fun with his awesome looking army, or add Skitarii support formations to fill those holes. That would be a lot more fun and interesting than having a hard cap because you'd be taking Skitarii units precisely because you needed them for some reason - not because you were forced to.
Some ideas on how it could be possible to go on about it it.
One of the Titan weaknesses is their sorry state of AA. Downgrade CML AA capability even more (to one shot?) or remove it altogether, and make the Princepses rely on ground flak formations for AA coverage (say, Hydra batteries). There's always Thunderbolts too but you can't hold ground with them.
Introduce a ground based Supreme commander Tech-Lord. Leave it available for Titans - on Emperor class only. Thus it's a hefty investment and players will have it available but will also have a cheaper, easier option outside Titans.
If Reavers are so good, then just up the price of the damn hulls already. Jesus. I don't think we even need to stick to the same prices of Titans as they are in Marine and Guard lists. The balance equation is entirely different in a full Titan list. So price increases should never be out of question.
In any case if you have Skitarii formations perform some relatively important roles that can't be done by Titans equally well people will either take those Skitarii formations or suffer disadvantages.
And that is the more Epic way of balancing lists
