@DPLOnline,
So you are mainly here for inputs for your upcoming trading platform! Hmmm. Nice!
So just to tell what you are interested in - see every trader has its own style of trading, so the requirements will vary. Chart guy will as you something and technical something else.
As for me, the first thing is "reliablity" of technical infrastructure. There perhaps is no other profession, not even banking and nuclear reactor, which is dependent on "near" 100% reliablity of glitch free connectivity like trading markets. So first you ensure us about this aspect because all is dependent on it. We have learned lessons from our bitter experiences. Please tell us about your servers, systems, lease lines etc.
About features, I don't have a long list, I mainly trade futures and never hold them for more than 4 days; but I think Interactive Brokers like BO will be beneficial for all, coupled with features like GTC orders etc.