That's a tough choice. If you're willing to do the necessary routing there are LOTS of cool options.
I agree that the Bronco stock ceramic Strat pickup is very poor, so there's definitely some room for improvement.
The first thing I would rule out are Strat-sized GUITAR rail pickups. I know some of these are popular for Broncos, but I tested this with very poor results. Guitar pickups are voiced for... guitar. I found them severely lacking in the lows and far too mid-focused. It's a matter of personal taste I guess, but if you're willing to do the necessary routing it doesn't make sense to install another Strat pickup IMO.
Then you will have to consider the string spacing. Some pickups don't play nicely with Fender string spacing. ThunderTron/FilterTron-style pickups made for Gretsch spacing is an example of this. The outer strings will end up outside the magnetic field and sound weak. The rail versions of FilterTrons might work better though, I haven't tested those. Bi-Sonic/Darkstar pickups are also designed for a narrower string spacing. I have never tested a Bi-Sonic on a bass with Fender strings spacing, so I don't know if these are more forgiving with regards to string spacing than FilterTrons, but I suspect there must be a reason why Curtis Novak offer his BiSonic/Darkstar pickups for Fender spacing as an option.
The next thing I would think about is whether you need a hum-cancelling pickup or not. This could limit your options a bit.
Then I would consider how much money you want to spend. To me it doesn't make much sense to spend more money on a pickup than on the bass. The tone will probably improve, but bass itself won't really get any better. I'm sure Broncos can vary a lot in quality, but mine was extremely poor. It had a rather flabby, soft neck, which caused problems with an uneven response along the neck and severe fret buzz even after levelling/crowning the frets and setting up the bass to perfection. No pickup can fix a poor quality neck. If the bass plays nicely with your preferred strings it might be worth upgrading the pickup. If not, I would think VERY carefully about this before spending a lot of money on an expensive pickup.
Finally I would also think very carefully about whether this will be your main instrument, or a cool second or third (or 50th...) bass. If you want to transform the Bronco into a good (hopefully) allround bass, I would go for a tried and tested, hum-cancelling pickup. For me personally, that means a "vintage voiced" P-bass split coil.
If you want something a bit more out of the ordinary and are willing to spend a bit on money... Maybe the Curtis Novak H22 gold foil? Great tone, and it looks very cool! It's not hum cancelling though. You should also be aware that even though it's supposed to be mounted directly on the body with "little to no mods" this will very much depend on where you position it. In the bridge position you will most likely need one with a riser (you will also need to do some routing/drilling for the pickup wire). In the middle position (basically in the position of your existing Strat pickup) it will probably be at roughly the correct height when installed directly on the body, but you will most likely need to route the pickguard and also expand the existing pickup cavity a bit to make room for the wire. When installed closer to the neck the body will definitely need a tiny bit of routing (in addition to the pickguard), maybe 1-2 mm into the body, or the pickup will get too close to the strings. Still, if you're willing to do the routing you can of course also suspend the pickup from the pickguard and make it height adjustable, which will give you a lot more flexibility.
You should be aware that the H22 pickup has an extremely high output. Much higher than say a "standard" P-bass pickup. I tried to pair a H22 in the neck position with a P-bass split coil and the H22 was WAY to powerful to make it work. I had to use a DiMarzio Model P (which has a considerably higher output than traditional P-bass split coils) to get an acceptable balance between the two pickups. If you're only gonna have one pickup this will obviously not matter (unless you want it to have a similar output to another bass, which can make life much easier if you're gonna change between different basses on stage).
Anyway, the H22 is just one suggestion out of TONS of other options.