Employment rights.
As Michael Ford has explained here - www.tuc.org.uk/sites/default/files/Brexit%20Legal%20Opinion.pdf
without the EU, a future UK government would have 'pretty much unconstrained freedom of action' in relation to employment rights.
Anybody who has read some of the government's Employment Law Reform consultation papers on the need for 'flexibility' etc (i.e. greater freedom to sack people cheaply and cut their wages) will know that this government has been constrained by the EU from repealing employment laws it regards as 'burdens on business'.
Subject to any obligations linked to future trade deals, what a future government does to employment rights will depend entirely on its political make-up. And the conservative Brexiteers have been among those arguing for the severest deregulation.
The Working Time Directive, for example, has been fingered by 'Open Europe' (a body whose stats are quoted with approval by Michael Gove) as "costing the UK economy of £4.1 billion a year and the benefits are unknown".
Well, one of the benefits is your four weeks' holiday a year. Before the Working Time Directive, hundreds of British workers didn't get paid holiday. Hardly surprising, when you think about how an entire industry has since grown up around finding ways to avoid or limit rights such as paid holiday.
There is a myth that employment in the UK is heavily regulated. In fact, we are already one of the least regulated of all the developed economies(unless you are talking about industrial action rights, in which case, we are at the other end of the scale, and the government is falling over itself to introduce increasing amounts of red tape).