What most of clients expect in Addons of VPS? RAM, HDD, IP, cPanel and....??? Suggest me some more options so that it would help my clients to go for some more features with VPS.
While some (lots) of options are available via cPanel, you might want to provide some of those services separately, if the user don't want cPanel - backups, security, bandwidth-boosts (unless you have dedicated bandwidth pr. server) etc.
Seriously depends. Do you want Linux or Windows. Each OS takes up a different amount of room, etc. To me, speaking as an experienced Sys Admin, I look for at least 20GB space on a Windows VPS, or 10GB on Linux. RAM - 512MB for Linux, 2GB for Windows. Bandwidth - I've gotten away with 10GB before. Yes, 10GB.