Once his presidency ends, but Trump stays around in politics, what sort of thing(s) would he involve himself with in this regard?

An adult president would use his influence to support the party and candidates and the republican platform.  But trump isn't an adult.  He doesn't care about other candidates, as was evidenced by his poor support for the GA runoff.  He may whine and complain, but not much else.  And if there's money to be made in a new media platform, he might get involved, assuming he isn't under house arrest or in prison for his crimes. 

Apply for the position of a private dog catcher, since he may be barred from holding any government office; if convicted in the Senate and disqualified in the second round of votes.  

Democrats need to kill off Trump once and for all, Or else Trump will become larger than life.

Find a media platform to spew hate, insults and conspiracies