You know, of all the possible magical abilities and such, I think illusion magic could work for Trixie. Or other types of magic pertaining to showcraft. Changing colours would be up that alley, I think. She's not absolutely inept, but only skilled within her talent area. And if that makes you doubt anything, no, Twilight Sparkle trumps her even in her talent area if she has to. She just doesn't need that. That said, i't still pretty much Trixie. Well done!
This would presumably be young Trixie, before she developed her awesome powers. I always imagined she started out a normal pony but developed her powers slowly, first starting off a charlatan, then slowly becoming an unnaturally powerful unicorn without even realizing the power she truly wielded. She speaks highly of herself, but deep down she's overly critical and judgmental, which is why Twilight's power irritates her so much, because it just confirms that she's "weak" (Or at least she thinks it does). Because of this, I usually have her being much older than Twilight, about 10 years, because she acquired her power through experience, rather than academic learning and raw talent..
Anyway, that's a short little profile for Trixie I usually use.
Hmm... Well, I don't see Trixie as exceptionally powerful. The thing about unicorn magic is that it appears to have two discrete dimensions: one is the area in which their magic normally manifests itself and is at its most effective, determined by their talents, and the other is the maximal amount of power they can command. For instance, Rarity appears to be very powerful, but not like Twilight, because, even in the hypothetical case of her power not being far behind, the other dimension of her magic is still mainly focused around cloth, gems, designs and appearances. When freed from that focus in Magical Mystery Cure, she displayed some very impressive weather manipulation, powerful despite being unskilled. In this respect, she even outdid Trixie's Alicorn Amulet-empowered magic, without having any external power source herself. Trixie's case is different. Her focus area is illusions and things that can be used for performance of all sorts, such as magical fireworks, summoning and either teleportation or temporary invisibility (or both). When performing that sort of magic, she is at her best. However, in general terms and outside applications, the overall amount of power she can bring to bear is average at best. I do agree that her "I'm better than everypony" attitude stems from dissatisfaction with herself and her own abilities. Perhaps this is why she was so angry with Twilight: she's simply more talented by nature. It's "Why her and not me!?", it's envy. As for her being much older, I don't think it's "much", but I do think she's a bit older than the Mane cast. She had the time to finish or otherwise leave the Canterlot school (she did attend it, after all, but we don't know how well she did, and she certainly was not on as high a tier as Twilight with her personal Celestia mentorship (Twi probably didn't even attend normal classes)) and then travel around for a while. Several years at least. It may not be 10, but I could easily see 5 or so.