Making a game on a zero dollar budget? It's easier than you'd expect. The Construct engine is probably the easiest engine I've found to use, and the most readily accessible to someone of talents (I am, however, attempting to learn about other engines). Also, I enjoy making Abstract grow from a simple idea to a full-fledged game.
That being said, as I am the only developer of the game, I can do whatever I want to the game, and there are no major strings attached. So, as a result, once the main singleplayer campaign is finished, I will be open to adding new modes of play (I already have planned a make-your-own-challenge-room mode).
Also, those of you who played the demo may be wondering if there will some type of weapon besides the default left click attack. You will be glad to hear, that a progression system will be incorporated later on in the game's development. However, it will not actually be necessary. As a fan of old arcade shooters, I intend for it to be entirely possible to complete the game without any sort of special upgrade. This however does not mean it will be easy. On the contrary -- it will be a feat of skill to complete the game with only your initial weapon.
As to upgrades, I've already had thoughts for several that would all serve unique purposes. I will be looking to allow for several play styles, and not every level in the game will be simply a conveyor belt of enemies coming at you, down a predetermined path (I guess you'll have to just wait and see).
But first and foremost, as always, I will put functionality before shiney. This game will one day have cutscenes between each level. One day it will have custom music. One day, it may even have some strange form of multiplayer. But unfortunately, today is not that day.