I am not really a fan of these type of games, but I can see that it may be quite attractive and enjoyable for others who play shooter games.
Although, I think there are alot of things to improve on, the gameplay itself is already quite smooth for a game in its early stages of development. Quite Promising, Indeed!
On to the Main note:
The problem I have with this, is that for me just adding a combo system to the mechanics would not be enough to separate itself from other similar shooter games.
I'm eager to see what would be Strike Buster's UNIQUE SELLING POINT that would set itself apart from the sea of games of the same genre.