I'll assume B2B. The hardest part of starting something, generally, is getting the first 10 customers.
Which industries do you have personal connections to people that could be the first 10 customers?
Alternatively, which B2B products/industries are you yourself the customer, where there's a product you personally pay for, that you see how you could make it a lot better?
> Almost all industries are underserved by software.
Theoretically true. But practically some markets are already saturated or difficult to enter. For example, data warehousing markets, very difficult to enter.