I don't know about everyone else but I'm of this idea that Darth Mauls death was an unfortunate incident and wasn't supposed to happen. Darth Maul was supposed to kill the Queen and he was supposed to kill the two Jedi protecting her. That was his mission and he wasn't sent there to get "killed off" so that Sidious could take a new apprentice. The fact is that Anakins potential was unknown to Palpatine at this time so he couldn't have planned ahead. Maul was vital to Sidious plans. He was a killing machine and if I for once will follow the EU leading up to the Phantom Menace, Maul was a very effective killing machine taking out several people that Sidious thought stood in his way. Even if we disregard the EU, it's still pretty clear that Darth Maul is someone very important to Sidious and that he's effective in his work. So why would Sidious want to kill off such an important underling? He didn't, it was all unforeseen.

So that's why I believe that the untimely death of Maul was a blow to Sidious plans and it was only through the appearance of Anakin that Sidious could continue with his plan albeit being forced to change it a bit. Sidious had to be more manipulative and take more risks to succeed with his plan which made everything more complicated for him and which could've cost him his life. If Darth Maul had still been there, everything would've been more streamlined. If we try to picture the scenario that Maul has now killed off both Qui-Gon and Obi-Wan. He goes after the Queen and kills her and the soldiers helping her. This way Mauls secret identity is still maintained which was Sidious plan from the start. The Jedi would be surely unsure and unnerved at what has happened which gives the Sith an advantage. An advantange they lost when Maul was killed and Obi-Wan could report back to the Council on what happened. Later on I believe that Maul would play apart in the Clone Wars, most likely assuming the role General Greivous, meaning that Maul would be a Jedi hunter. It makes sense, doesn't it? It's a well known fact among Star Wars fans that Maul more or less lives on his hate against the Jedi and it is what drives him, so here Sidious would have the ultimate weapon to kill Jedi with.

The more I think about, the more I see that Grievous as an emergency solution so that Sidious plan would work. Greivous is all machine and that's what aids his fighting skill. It's impressive but when faced with a Force user, Greivous has difficulties. Dooku isn't a killing machine, actually for a Sith, he manages to keep his emotions hidden and doesn't utilise them the same way Maul did. Dooku is the civilized face of the Separatists and gives their cause credability. If they let Maul hold center stage, that credability would be shot down.

So, would Darth Maul be the scourge of the Jedi? Could it have been Darth Maul marching up to the Jedi Temple? It's hard to say, since this hypothesis deals with so many variables but I believe this could've happened. I'm pretty sure that Maul also knew what Sidious was up to, with his: "Soon we will reveal ourselves to the Jedi. Soon we will have revenge", which is a contrast to Dooku and Greivous who believed themselves to be fighting a corrupt government. Maul knew that the Sith would take over through manipulating a war and not by fighting a war. The only thing I'm sure of is that Sidious' plan would've worked easier with Darth Maul alive and that he was much more important then Greivous or Dooku.