how about this gametype.. instead of chase the ice cream car like in gta3, 
You steal the SecureCar from the bank, and you have to deliver it to your gangs base without getting it jacked from u or blown up... and u got like 20 people chasing u with heli copters and vehicles and the Cops try to prevent it from being jacked... i just got done playing this in the MTA offical server and had a blast doing it! Also how about Gang Raids? its simple... you spawn as a gang member of your choice, and guard your base from the attackers. Earlier i got about 8 guys rounded up and we raided the airport with the mexicans in it 
theres so many gametypes you can make for this game... I can go on and on 
BTW: i like Magus idea about 3 armored cars and 2 with nothing in it... this would be even better to add to it because its impossible getting away from 20-30 guys without a scratch