My guess is that Republicans will keep the restriction that insurance companies cannot deny pre-existing conditions. However, without the individual mandate that would bankrupt every insurer within short order. Avoiding that outcome is, afterall, the entire function of the mandate.
So what I expect to happen is that Republicans will cover the insurance companies' losses with some kind of new Medicare/Medicaid programs. However, over time (or maybe in short-order) it'll effectively be a blank check for insurers, funneling money from the government into private industry. Typical of similar Republican-designed programs, it'll be an unfunded expense adding to the debt. (The GOP is still the party of Dick "the deficit doesn't matter" Cheney.) Though, I have no doubt it'll become fodder for more anti-entitlement rhetoric from the very same politicians and pundits who supported the program.
I've already invested heavily in some insurance companies with proportionally large Medicare exposure. Unlike most health insurers, those companies actually took a nose dive yesterday, presumably based on the intuition that without Obamacare the Medicare/Medicaid spigot will shut-off. So, to my mind it was a doubly good investment, as companies like UnitedHealth were already up double-digits, making them less appealing to me (too much upside gone).
It's definitely a gamble. Who knows what Trump will do. But many Republican leaders have already stated that they don't think it's politically feasible to remove the pre-existing condition limitation altogether. Likewise, they've made it clear the mandate will go. The logical way forward would be to back-door kickbacks to insurers through Medicare/Medicaid. And unlike Obamacare it's much more likely to provide out-sized profits for insurers.
I suppose one alternative is just to allow people with pre-existing conditions to get on Medicare. But I think that's too openly pro-entitlement. Republicans like to grow entitlement programs like everybody else (it's what gets votes), but they prefer to obscure things so their policies and budgeting isn't openly contradictory with their rhetoric. (That's fair game, however it also makes it easier to hide deficit spending. When spending is out in the open the electorate is forced to make hard choices; when you hide deficits the feedback process breaks down--the electorate isn't forced to reconcile the specific costs of the program with the specific benefits.)