People can give you a zillion different suggestions, but until we know :
A) What you don't like about the bass
B) What sound you are TRYING to achieve
C) What the rest of your rig is
....well, then we can't be truly helpful. It's kinda like someone saying "I'm getting a Honda Civic...what should I do to it ?" (Do you want better mileage, power, comfort, a certain look, etc, etc....??)
Agreed.
That being said, I did a few upgrades to a Squier Affinity P that I found on CL, so I'll list my mods as "food for thought".
1) Mighty Mite maple/maple Jazz neck, as the standard P-neck is a bit too wide for my liking. Besides, the PO had pretty much hashed the nut on the old neck at some point, and the rosewood board had a few dings in it - it didn't take much to convince me to part with the dough for a new neck. Cost: $100.
2) Stock pickups were less than exciting. I was going to try the GFS PB Pro PLUS P-bass pickups from Guitar Fetish, but I scored a set of nearly new Seymour Duncan QPs on CL for just under retail. Cost: $60.
3) Since the volume control was scratchy, and the tone control was either on or off, I replaced both pots with CTS 250K units and a new OD cap. Cost: $18.
4) Stock Squier strings (which looked like they had been on the bass for years) were replaced with a set of D'Addario ECB81 XL Chromes flats. That's what I had lying around, so that's what I used. Cost: $30.
5) Sold the stock Squier neck on CL, so toss in a $50 credit.
6) I've been eyeballing a new Gotoh 201 bridge, but I haven't yet pulled the trigger. The stock bridge works fine, even if it is ugly. A possible future upgrade? Maybe? Cost: $0.
I have around $250 in the bass and upgrade parts, and I can say that I'm certainly happier with it now than when it was in stock form. Most of the mods were made to make it a nicer player, and all the stock bits (pickguard, knobs, tuners, etc.) were re-used when possible.
Is it nicer than a MiM? Well, I'm not going to say, because I'm biased. But I'm happy with the bass for the money I have in it. Plays well, sounds good, and if it was stolen, I'd build another like it. YMMV.