I am considering a move from developer to a solo consultant specialising in something. Just looking for ideas of what types of specialist consultants exist out there and what is marketable and profitable?

Rather than just specialising in a particular technology, programming language or framework I am looking to see if there are opportunities to specialise in things like:

- Optimising sales funnel (shout out patio11) - Data cleansing - Improving Analytics - Upgrading legacy systems - Development for specific industries etc

Curious what specialities people are having success marketing themselves as?

Thanks!