Most job contracts contain some form of an NDA and a non-compete clause. Both may apply when you start making "better widgets" than your former employer. Review the contract you signed when you got your job, preferably with a lawyer.
Starting a new company is easy, the hard part is getting the revenue to keep it going. A lawsuit will cost you time and money and is a great distraction from doing business. Even if you win the lawsuit, the company may go under due to the legal costs. I would recommend making "something completely different" to avoid a lot of problems with (soon) former employers.