Any Republican chosen by the Republican National Committee would lose the support of the “deplorables” (previously referred to as “Reagan Democrats”, i.e. working class whites). None of the Republicans acceptable to the Country Club Republicans would be tolerable to the Trump supporters. To unite all Republican factions would require a socially conservative tea partier who wants to build walls and persecute non-Anglo-Saxons, and there ain’t such a critter.
If a Republican wants someone with traditional small government, pro-capitalism principles, they should support Gary Johnson. However Johnson’s strategy was to carry a few states and hope for a close enough election that it has to be decided by the House of Representatives (first time in almost 200 years) and to win as everyone’s second choice – but a Trump “blow out” prevents that.