Been thinking about this for a while - ever since primals was introduced actually - but wouldn't this be an awesome idea to:
==> reinforce the idea that Primals are great fun and rewarding
==> having a primal drop when you first complete your solo GR70. The primal should be of the same type (random which slot) as you've got equipped !
Thus it would be really rewarding, since it would be an item you actually used and would likely keep, rather than getting The Grandfather or some Gladiator Gauntlets (incidentally the first primal that dropped in the clan this season).
PS. to be clear: If you have say Vyr, Tal Rasha and Nemesis on, you'd get one of those equipped items but primal, with random rolls on them, or perhaps even with same stats as you currently use
PS/2. Another thought is to add a Cube recipe to reset rerolls on items...