I have a squier tele custom II, with a humbucker in the bridge and a P90 in the neck. Generally the tone is quite dark, really lacks top end sparkle. Now, it's a squier, and the hardware is cheap (pots have been replaced - 500k). Would a decent bridge brighten it up at all? (It's a short bridge, not the long one with the pickup hole).
Revert it to traditional Telecaster style with two single coils! Much better result all round anyway. ;)
A new pickup will brighten it, sure, but a humbucker is always going to sound darker than a single coil. I doubt you'll ever get the characteristic sound of a traditional tele because a lot of that depends on the physical arrangement of an overwound sc pickup attached to the enlarged bridge plate. You could try a P90 though. Just make sure it's going to fit before you buy.
If you're handy with the soldering iron, another thing you could do is add a bleed capacitor on the volume control like in a traditional tele. The classic twang sound is got by rolling down the volume a bit so that the bass is cut, while the bleed cap lets the treble through. Diagram [url=http://www.specialtyguitars.com/kits/tele_diagram.pdf]here[/url].
BTW if you're looking for old school tele sound without spending big money, you hear a lot of good things about the Squier Custom Vibe teles. There was also a G&L ASAT on here for £250 - some would say that's a better guitar than the Fender.
I bought the guitar because it was a cheap way to get a tele pre routed for humbuckers! I couldn't afford to buy another american, so I went totally budget. Even the neck P90 sounds dark. It could just be I got a bad plank of wood, but I was just wondering if there would be a difference. Next step is to take the tone pots out entirely and see what happens :)
I'm handy enough with an iron to totally rewire a guitar, and to put a DS1 INSIDE a strat, so any electronics work is cool. I may be spoilt for high end with that guitar having a buffer so close to the pickups, it's a big difference! If cutting the tone pots makes a difference, I'll try the treble bleed anyway. Might change the bridge for the craic anyway though!