I always loved IC since launch. Been trying to farm telvar again. But I'll tell you the biggest thing that makes even me not want to be down there.
1-lack of anything to do in sewers. I think they should add three flags per portion of the sewers, increasing telvar gain similarly to the city flags but only for the zone ur in. I think their are seven zones in IC? So three flags per zone. Also no NPCs on flags please....or you can make the boss kills dependent on whether you own an area of the sewers. You kill the roaming boss you own the area.
2-the risk is too high compared to the reward. I'm not opposed to risk, but the setup we have is too heavily leaning on sneak and stealth...nightblades have the greatest advantage in the gold to be made in the whole of IC. Ur either a solo NB, a solo tank, or a zero. If you're outside of that. Expect to lose a ton of telvar, I think lowers lost stones to maybe 1k when you die. That way ur encouraged to still go for telvar, but if you die to gangs or ganks ull only lose a small portion. Allowing you to carry more and not sweat too much. This IMO is the biggest change to bring players to IC, don't even have to change anything else if you don't want to.
3-add flags to each upstairs district, and also dramatically lower the amount of telvar gained from flag ownership, instead of 33% make it 10% or 50% for only that zone. You don't need to add more flags...just change the numbers around a bit. You want to kill the boss in this district, then own the flag to maximize telvar instead of all 6 flags.
These changes right here will see IC explode with populations, some players won't want it...they rely on the gold income. But it's an unhealthy environment as it is. These changes are super easy to implement. Even if you don't change the flags or the amount of flags, just messing with numbers for telvar gained and lost will make zeni win