The 1/3 system on its own will not work because Clans can still have people inside grinding troops to replace any losses. With enough grinders you can easily make certain fiefs unconquerable.
On the other hand we should not want to end up with a system whereby Towns can be taken in 1 assault easily and regardless of the number of troops inside. It is already possible with flag capping but it shouldn't be any more probable because then we go back to a strat where whichever big clans happen to be most active will stomp on everything in their path (like Strat 3).
What we need is a way of freezing fief garrisons under certain circumstances so that fiefs can be ground down naturally over time with 1500 man armies. Then the defender has to either sit back and take the assaults, which will come at the attackers convenience, or they have to sally out and leave their defenses to break the siege.
I already made a suggestion with a pretty long explanation about a possible solution: Basicly you need to besiege a fief with an army that has at least the size of the fiefs army (yes, besieging will be expensive, nothing comes for free), and this should block all troop&equipment transfers to/from the fiefs (this also includes the equipment bug getting fixed so you can use all your gear all the time), the only thing possible should be selling items so you can get rid of minused stuff. Once the fief is besieged attackers obviously can put in as many/few troops into an assault as they want, though they might want to end it quickly due to the high upkeep.
Ofc the army should be attackable - either by a counterassault from the city or by attacks from the outside.
Attackers from the outside should be able to breach the besieging armies position when they win their battle and reinforce the city with what they have left afterwards - an idea i like for this is that the besieging army is "spread" around the city. So if there is as many besiegers as besieged, the attacking army has to fight the same amount of tickets it has. So if you besiege a fief with 10k that has an 10k army, an attacker with 1.5k has to fight 1.5
But if you besiege it with 20k, an attacker with a 1.5k army needs to fight 3k.
Whatever survives the battle can reinforce the city, sth like this if i remind it all correctly.