Around here I usually see (and give) the exact opposite advice.
Making a whole new game using an existing one as a base is a silly way to set about things. Probably still even if you have a high end tool to edit the game with.
A remix, a mechanic tweak, a boss rush, a graphics tweak, a music tweak, a control tweak. All good things you should absolutely do as a hack. Full new game, possible but probably harder than making a whole new game.
"team of people ten years"
If this is a weekend hobby (and if you think fan games are a good idea and are using game maker* it probably is) then that already sends timelines up (see also ROM hacking -- not so many here do things as a retirement, unemployment or school holidays project and this is why it can take ages), if as speculated you are also learning game design/code on top of this then it is a miracle anything gets released at all really, if multiple people are in that boat then you get to wait on others to learn and do, these sorts of things take longer to get the ball rolling on than leaving a pub/bar when you have 5 different groups to say goodbye to and even with all that a proper go at this is not a short affair -- all those game code jams don't produce long games unless they use procedural generation as making long games takes ages.
*I have heard of many devs thrashing out a gameplay idea in gamemaker, unity and such (might be resource intensive, somewhat restrictive/weak in what you can make it do easily and not look so hot but they do allow for rapid development if you can handle it) to see if it works so I certainly can't say all there. Not to mention Unity has really upped its game this last year or two.
That said I have no real reverence for fan games. Just seems like a bad idea from a legal perspective, and a stifling one from a game design perspective. Make a clone or a homage or something, and then figure out what is wrong with the game you have cloned and fix that and now we are talking.