Creating a new shooter, platform whatever is not copying a specific game.
However starting a project to create Sonic, Chu-Chu Rocket, Golden Axe etc for the Amiga is a bad waste of resources.
I am positive you can come up with a new game in any genre that you'd enjoy playing without the need of breaking copyright laws and copying others' work.
My point is copying a game is pointless, it might be interesting strictly as a geek project (for a few dozen people at most, but not the end-users), you might raise some wows, but your work worth of years is wasted as pretty much everyone just deletes the game after having seen what it is.
So my call is to avoid any confusion: make a new game, you don't have to come up with a new genre, but you should be doing a new game that is not a port or a straight rip-off.
As an easy start surely you can think of a tiny little addictive something as an exercise and to get the group work as a team, in preparation for a bigger project.
Even combining the best elements of games you like is better, than copying them.
Alternatively do a remake for an exercise, but add something to the game, graphics sound and game play wise all if possible to make it worth the effort.
Just doing something and wasting talent instead of spending some time, creating, cherishing and enchancing a new game concept (again: not a new genre, etc) is not very useful.
Take your time, discuss the concept, do a decent design, and once everyone likes it create a prototype without a real engine to see if it really works as a computer game, not just as paperwork.
Once you are satisfied do the real engine, the real graphics, real sound etc.
This is how it works, an amateur project shouldn't be amateur because of being unprofessional, it should be amaetur because it is not profit oriented and is being done for the fun of it. (just like sport was supposed to work a few decades ago)