Number one most important factor: Originality

It has to be different than just about anything else on the market. There are so many specialized baits already, that most categories are already near perfection. If you try to make a jointed bait, you'll have a hard time trying to make a better one than the slammer. Paddle tail, the osprey and rago trout are tough to beat as well, and there's also a ton of baits in that category currently on the market. Yours may get left in the dust if it's no different than the other dozens of paddle tail swimbaits currently available (which are tried and true). 3:16 has some pretty good original designs, that's what I like about mickey, he's original.

Word of warning- if you're going to make a swimbait to sell commercially, first of all make sure you're not stepping on anybody's toes already making swimbaits. Secondly, make sure your bait will catch fish, not necessarily fishermen. How pretty does an MS Slammer look anyway? Not as fancy as a castaic hardbait that's for sure, but it's the most productive hardbait that I know of. If it catches fish, the fishermen will follow.

Your design is going to take a lot of original thought to present something that will catch fish consistently, warrant its price tag and give people a reason to buy that one over the other dozens already available. I would suggest spending a lot of time throwing swimbaits, a lot of time talking to guys that throw swimbaits if you can, and knowing the ins and outs of swimbait fishing and where there's room for improvement and addition.

With all that said, here's a few things I find important in just about any swimbait:

Hookup percentage: ms slammer with gammie trebles probably has the best hookup ratio of any swimbait. The more trebles the better on swimbaits. You don't carolina rig a trout bait, there's only a couple situations where you need a weedles trout bait, and that niche is pretty well filled (mission fish). The vast majority of the time if you're fishing swimbaits like you should, it'll be in open water and you should be more concerned about hooking fish rather than getting snagged.

Size: if you're replicating a trout, 7" should be the smallest you go. I'd say the largest would be 14", maybe bigger. 12" is a safe bet for sales.

Motion: good action is key. In my opinion the megabait charlie is a great example of what not to do... Make your bait not swim like the charlie and you'll be ok :D Best action award would probably go to the stocker trout. Trout in real life just kind of cruise with flicks of their tail. You want your swimbait to exaggerate their motion like it's struggling, or looks like it can't burst off very fast if it's ambushed.

Color: this plays a minor role. Size and motion seem to be the stand bys. If your swimbait is colored somewhat like a trout, it'll be fine. This is the part where you can catch more anglers than fish. A pretty paint job is fine and dandy, but I guarantee a bare wood colored slammer will still catch as well as the day it was painted. I used to have a slammer that was so chewed up it looked nothing like a trout color anymore, and it still caught just as well.

A little more than $.02
-Brian