One thing I really dislike about most MT4 brokers is their "10 pip rule" - that being that pending orders orders must be at least 10 pips away from market price. ECN brokers actually allow you to place pending orders inside the spread.
If you use a system that buys/sells on the break of a previous candle high/low, or even if you just want to adjust your S.L. or T.P. by a few pips when the market price is near your order, you can't do it - you have to stare at the screen with your finger on the trigger ready to enter/exit if the price you want becomes available. I often trade flags & pennants so the situation arises quite frequently. It's a pain in the @r$3!
What I think would be most useful is an EA that could assist manual trading by being able to place 'virtual pending orders' and when your price was hit the EA places a market order with slippage control parameter. This would allow S.L's & T.P's to be hidden from the broker too. If it also had some trailing stop features all the better. I am more of a fan of moving S.L's in jumps - ie if prices reaches A move stop to X, if it reaches B move stop to Y, if it reaches C move stop to Z.
Anyway that's my idea. I'm surprised that an EA that does this is not already in existence. If you know of one please point me in the right direction.