Most popular kitplanes could likely be the basis for a certified plane. Sure, you'd have to use TSO'd parts everywhere, and probably do some engineering and development here and there (for things like spin resistance), but that's all stuff every other OEM has gone through when bringing a new model to market. That's why you see things like stall strips and dorsal fins (had they known for sure how much vertical stab they would have needed, they would have designed it that big in the first place.)
The biggest difference between a good modern kitplane and it's TC'd brethren is just the sheer amount of documentation and testing. A company like Vans does a lot of testing, and puts out a safe aircraft, but the amount of testing a TC'd plane goes through is mind blowing. That battery bracket you bent up out some heavier than necessary aluminum cuz you had it and didn't want to worry about it and is obviously overkill....yeah, they have to test it. And it's not well the battery weighs 26 lbs, times 9 g's, equals about...he Bubba, you weigh about 240, right? Come sit on this quick. Nooooo. First you design the part, do a detailed drawing, and a company test. Next the engineer has to write a test report, which then has to be signed off by a DER, and after the FAA points out 3 esoteric AC's you didn't know existed (and the idea that AC's are "Advisory" is a joke, they might as well be FARs because the FAA is going to make you follow them) and you've redone the entire process starting from the beginning twice, they may sign off on it. Then you build new parts, which have to be conformed by a DAR. (Conformity is an in depth quality inspection to ensure it meets the proposed design with a lot of research and paperwork associated.) Next comes the test setup, which is conformed as well. Then the actual test, which has to be witness by a DER. Then the data is recorded. Now a test engineer reduces it and writes a report, which is checked and approved, and finally sent to the FAA. If they are okay with it, and if it isn't a federal holiday, and if the planets have aligned and you have sacrificed the required number of goats, they will sign off on the report. Congratulations, you have a TC'd battery bracket. Only 3241 parts to go.
More than one aspiring aircraft manufacturing has successfully run the above gauntlet and TC'd an airplane, certificate in hand, only to find out that they are now 1/4 of the way there. Getting a Production Certificate to actually build the darn thing makes getting a TC look like child's play. Every part needs planning, every tool documentation and a procedure for checking it. Putting in rivets? In the glory days craftsmen just pounded them home. Not anymore! Now you need documented procedures and acceptable parameters. Spraying primer? The procedure needs to lay out material type, number of coats, thickness of coats, how you are going to check the thickness, how to calibrate the thickness tool (yes, there are tools for this, they are ridiculously expensive, and you'll have to buy one), etc.... And then there is the paperwork. PAPERWORK. P-A-P-E-R-W-O-R-K! Reams upon reams of it. Work orders, inspection records, certs, MRR's, and on and on and on. When the weight of the paperwork equals the weight of the plane it's ready to fly. (Your laughing, those of who work at OEM's aren't.) And all of needs to be kept....forever! Why, because the MIDO comes in quarterly and inspects the place and looks for it.
I could go on, but you get the point. If you get a chance, take a tour of a small airplane factory. It will open your eyes. Whether or not all this is really necessary is a debate for another day. But it is the difference between a lot of good kitplanes and their store bought counterparts.
DEM