I don't know if any of those things are planning or even if it's possible to implement, but could be nice addictions:
1 - ''Black market'', possibility to sell and buy drugs, with chances to get stars/arrested if someone u try to sell/buy is a undercover cop.
2 - Drive thru in every possible fast food restaurant, like Saints Row.
3 - Casinos in the Big Cities, and illegal gambling ones on interior cities.
4 - Nightclubs in every city(at least the big ones), with a minigame like the champagne competition in TBOGT.
5 - Ferry boat like LCS
6 - LC/LCS cars in Carcer and MLL
7 - Possible to turn on/off the engine and the lights like IV.
8 - Mental disabled and assassins in Aylum Happy Volts/Dixmor and Darkwoods Penitentiary(or at least shouting screaming sounds).
9 - Wooddale Community Enclave Daniel's Lamb house as a safe house.