[danielvf]

C# and asp.net. The tooling will stay roughly the same. Many of the concepts will stay the same. These is plenty of work available.

[brudgers]

Not only is there work available, by staying in the Microsoft ecosystem, existing clients for Classic ASP are leads for upgrade projects.

If picking an outside technology, PHP might be a reasonable addition to Classic ASP because it often fills a similar niche.

Depending on goal specifics, ASP.NET MVC might be a better option than webforms...even though webforms are a simpler abstraction when I look at them.