I 'd say not to stick too much to space invaders, for a group like yours to suceed, you are going to need a lot of MOTIVATION and DYNAMISM between the group. I think those are the key, as the project leader usually keeps working alone after something, because others didnt feel motivated enough.
I think its kind of the same feeling a parent has towards his son
Discuss between yourselves what is the medium of everyone's decisions, do something feasible. An RPG is nothing special if you keep it simple.
When people say a RPG is tricky to make, its really about complex inventories, too many items to fill them, too many weapons and customizations, multiplayer perhaps.
Stick with me, if you had in your mind to start a RPG.
1 - Tile maps or sprite placement - Just use a premade editor and parse that file. Absolutely doable.
2 - Physics, add a basic quad collision system.
3 - Make easy to parse xml or anything else formats to specify your content
4 - Make a base entity, and derive it to player and enemy, and spawn some of those.
5 - Deduct hp, earn gold on kill
This may not seem a deep planning, but if you stay focused and work with your mind in place, each of the tasks is very doable in a few hours, depending also if you make wise choices on how to approach issues like the tile map format(thinking TMX as a good example for example, theres tutorials).
The numbers game is funny but once you do a few of those you certainly gain a lot of experience and something like becomes too easy for you, logically speaking. I bet you can do that game in 30 mins instead of 4 hours when you grasp the logic better
