Kinda like 
's ult
Initial cost is 20 mana, then 40, 60, 80.. etc
That way, she mostly uses her rockets for poking and not her continuous dps
So that she's actually forced to dps in minigun-form so that, you know, she has "counterplay" instead of hitting for potential 550% AD for 20 mana every auto
JustMyBassCannon12 votesBecause Jinx's Q is totally on par with an ultimate that has 1200-1800 range and a larger AoE.
/sarcasm
Altricad8 votesPretty Sure 
's rockets have the same Aoe as kog's ult.
You also have to take into account that ult is a skillshot while Jinx's rockets are click and point.
The range on Kog's ult is higher but it's damage is also much lower
JustMyBassCannon7 votesYou also have to take into account that it has a 1-second cooldown, works with Rylai's, does not require him to be in attack range, and while the damage potential per shot is lower, if it did not have the stacking mana cost, he would get to use it significantly more often than Jinx uses her Q rockets.
Kingopants1 votesYou also have to take into account that it has a 1-second cooldown, works with Rylai's, does not require him to be in attack range, and while the damage potential per shot is lower, if it did not have the stacking mana cost, he would get to use it significantly more often than Jinx uses her Q rockets.
Jinx rockets are shot roughly one a second as well, don't require her to be withing attack range (They increase her attack range), Apply Rylais (the explosion) work with Runaans and are not skillshots.
Jinx uses her Q rockets a lot.
JustMyBassCannon7 votesJinx rockets are shot roughly one a second as well,
Which means Kog'maw is capable of firing them as frequently as Jinx.
don't require her to be withing attack range (They increase her attack range),
That still requires her to be within attack range. Her attacking range increases, but she still needs to be in range to use her rockets at all.
Apply Rylais (the explosion)
No, because her rockets are an attack effect, not a spell effect. Also, more importantly, Rylai's is actually useful to Kog'maw, whereas it isn't useful to Jinx.
I'm glad you didn't go and mention Frozen Mallet, because that's both inefficient and less effective.
work with Runaans and are not skillshots.
This is true, but a minimum 500 range advantage between Kog's R and Jinx's Q is still there. And it only becomes larger. The fact that Kog'maw actually has to aim doesn't really detract from that as much.
Jinx uses her Q rockets a lot.
They're her basic attacks. Of course she uses them more often than Kog'maw uses his R.
Eedat1 votesIts actually better IMO. Kog'Maw's ult does little damage unless he's building AP Kog and jinx's rockets can do massive AOE crit damage
Social Justice 16 votesThe initial cost cannot be 20 mana if it's going to stack, unless it's something like 20+2X mana per rocket (X=number of rockets fired after the first)
Gubaguy4 votescan you not? jinx already cant use rockets constantly, if she does she burns all her mana and is a sitting duck. increasing the cost woudl mean increasing damage done per shot, and no one wants that either.
also jinx cant duel with rockets, she WILL lose a 1v1 if she tries becuase of her low attack speed withotu minigun, and even then rockets only help for 1-2 shots ina teamfight, or for long chases to get the last shot in.
Ulquiorra9953 votesJinx might be somewhat over the top now, but what you're proposing would send her straight to shit tier. Charge a manacost to an ADC for using their autoattacks so bad that they run out after 6 consecutive AA's? I don't see how that could backfire at all.
MInd you, there is a reason no one is using minigun in teamfights although its DPS is far more superior to rockets (120% AS buff vs 10% AD buff) - it has lower than standard range and Jinx has no on-demand mobility or non-delayed CC to defend herself that close.
My Waifu Vanilla1 votesExcept if you hit 2 people with the Rockets it is also a 120% damage boost which only gets bigger the most people she hits. A jinx with a zekes support at 100% crit and instantly win a team fight in 3 autos which is not hard if you have any sort of cc on your team since she still has quite a bit of as with the p daddy and infinity edge.
FantasySniper3 votesBut... she's already forced to DPS with Pow-Pow.
Pow-Pow is better at attack-move and DPS. Fishbone's only real use is getting in a couple of AoE shots at long-range.
If you want to "fix" Jinx, actually hit one of her strengths.
Quil Evrything2 votesnaw. just make them cost MORE mana. Way more. 40 a shot at least. Then she actually has to make a CHOICE between spamming rockets, vs using traps and zappy's.
Bob the Toastr2 votesMaybe make it more of a trade-off for the mana cost, like Riftwalk: each successive one costs 20 more mana like you said, but also does 10% more AD in damage (110%, 120%, 130%, etc.) and possibly even make the range scale each time like an extra 10-20 range per auto (to go thematically with her being more accurate each time she uses her rockets, like being able to hit the target more consistently after getting used to it) like 50, 60, 70, etc. at level 1 and 125, 145, 165, etc. for level 5.
Angelofchaos16x1 votesI think that it would need number tweaks, but it is a interesting idea.
Guy Fox Teemo1 votesIt's basic attacks... Gonna make it stack, make her rockets 700 at rank one and 150% damage and a bigger AOE. You seem to think she wouldn't be completely shit with these nerfs.
Dreadlocks1 votesTOGGLE ― FISHBONES: Jinx's basic attacks gain bonus range and each consume mana, also dealing 10% AD bonus damage and dealing their damage to all enemies around the main target. The bonus and splash damage are both factored into critical strikes.
as if this^ isn't already strong she also gets this...:
BONUS RANGE: 75 / 100 / 125 / 150 / 175
Why rito, AoE crits hurt, So does insane range.
Hayaishi21 votesShe shouldn't do 110% damage in AOE either.
The Bíg Ticket1 votesMake them not able to AoE crit, it is enough that she hits targets for 110% of her normal Attack Damge.
My Waifu Vanilla1 votesMy only problem with her Q is they can crit on the splash which is very demoralizing when mid game with just IE she can crit an entire team with a single auto because the rng gods said so. I also think they should update her Zap's indicator since the ability is deceivingly wide and longer than that line says but that is another problem altogether.