Congratulations on writing your first trading system. Starting out in C# is a good idea. It's a powerful language and many of the best software packages are based on CLR.
Tanyach recommended Ninja Trader, and I agree that this is a very good piece of software. I prefer WealthLab, RightEdge or...
It depends on your definition of open platform, I guess. Most platforms can be extended in some way, but for most consumer level apps you'll hit a wall quite fast. When you try to do something more complex you'll see very big differences between say MetaStock and TradeSignal on one end and...
My advice is to always favor open platforms. The more a platform can be extended and modified, the better. Closed systems with proprietary scripting languages usually paint a pretty small box where you have to try to make due within their rules.
I prefer .Net based applications which allow you...