Beside from his slaying of Qui-Gon and his subsequent death at the hands of the young Obi-Wan, I don't know many things about Darth Maul.
I read somewhere that Dooku was considering him merely a beast, but I suspect his opinion of being distorted, as discounting his predecessor is a way for him to increase his value.
So, how powerful was he? Aside from his Lightsaber skills, how was his Force mastery? Did he had enough potential to overthrow Sidious one day?
Darth Maul did possess some significant levels of Force mastery, but according to Wookieepedia was inferior to "true" Sith Lords:
in order to avoid breaking the Rule of Two, Sidious was only allowed to train the young Zabrak as a weapon; he was never trained to be a true Sith Lord in the same way as Plagueis and Sidious.
While his greatest strengths were his abilities with weapons (Maul was a master of the fighting styles Juyo, Jar'Kai, and Teräs Käsi, with training in Niman, and mastery of the lanvarok), Darth Maul possessed a number of notable Force masteries:
Maul displayed a reasonable amount of competence in telekinesis, utilizing Force grip and Force choke while interrogating Hath Monchar, and being able to activate a set of door controls behind him with a flung piece of wreckage.
While dueling Obi-Wan Kenobi, Maul used both Force push and Force lightning against the Jedi (this reference is from The Phantom Menace video game, which I believe makes it C-canon).
Maul also demonstrated a resistance to Force lightning, shrugging off a barrage unleashed by the Nightsister Mighella while hunting for Alexi Garyn.
Darth Maul was skilled in combining Force talents with mechanics. Sidious gave Maul the funds and schematics he needed, and provided his apprentice with a secret facility on Coruscant in which to work. There Maul constructed his signature speeder bike Bloodfin and the Dark Eye seeker droids. Using the Force mechanical skill mechu-deru, Maul modified the protocol droid C-3PX into an asssassin droid to guard his ship.
Also, while fighting the Force-sensitive Drovian Silus, Maul used the Force to overload a nullifier field that had been preventing him from using his lightsaber.
Darth Maul and his apprentice fought Darth Sidious in season 5 of Star Wars: The Clone Wars.
You can watch the fight here if you don't mind spoilers:
As you can clearly see,
Maul and Savage combined still get absolutely owned by Sidious. Based on this evidence, it is safe to say that Maul, while a very good fighter, was nothing compared to a true Dark Lord of the Sith.
I would state that Maul is pretty powerful. It is most likely inarguable that Maul was one of the best martial artists in the entire Sith Order. As stated, he was a master of Teras Kasi, the best martial art form against force-users mostly due to its usage of mental barriers. Maul was also pretty good with Juyo, and Niman, and from what we see in the movies, he is reasonably competent in Ataru due to his usage of acrobatics, and was able to defeat Qui-Gon, a powerful Ataru master(who had somewhat bad stamina).
When we look at Maul's fight with Savage against Kenobi, and Adi-Gala, Maul was able to -at the end- pull a starfighter using telekniesis, and pull it down toward the ground. While, in that era, there were most likely a lot of Jedi Masters who could probably perform a skil like that, it does show that Maul is a master of telekinisis, and is pretty skilled at it. Maul is also pretty good with light-saber combat, exemplified by the fact that Savage Opress was able to beat Adi-Galia in combat, and Savage was just a guy with brute force; Maul was able to beat Savage in the beginning parts of that episode specifically.
I've also looked at Maul's fight with Sidious(after Savage was killed), and Sidious fighting 3 Jedi Council members, and Maul lasted a lot longer than all of them(seconds apart, but still longer) except with Windu, who was an absolute powerhouse in lightsaber combat.
In terms of his capability to overthrow Sidious, no one execpt Anakin, or Luke can be able to do that tbh. Sidious was probably the most powerful Dark Lord in history, and Maul was just an Assassin, he wasn't a true Sith apprentice(something that I believe Sidious himself even mentioned).
I hope this answered your question.
Maul's training wasn't even near complete.
In his fight with Obi-wan and Qui-gon, he owned them leading them exactly where he wanted. Just one stupid mistake on Maul's part lost him the fight.
Maul's lightsaber abilities were a match to Sidious's, it's just he never refined using the Force in combat and often used it as an after-thought, which is why Sidious beat him.
When he did use the Force he could rag-doll some of the most powerful Jedi, and throw them across rooms and so much more. His problem was he didn't use them much but if he had more training he would have been one of the most powerful Sith
