This is not an easy one, but since i exposed myself, i will try to give an answer.
The ideal candidate would have a complete aviation curriculum: Single, twin piston, aerobatics, twin prop, RJ FO, single aisle FO or RJ capt, heavy FO, then maybe single aisle capt., or similar. This would equate to about 5000h.
As this is illusionary today, a good mix would be suitable. That contains sectors, preferably in different environments and ideally some jet experience plus definitely solid handling skills. This would equate to roughly 2500h.
Due to the very short training at the ME3, exposure to more than one of the former is essential. What i observed is that prop guys can get used to jets very fast, low hour jet cadets can have some good handling talent, locally constrained regional experts can adapt to new environments comfortably, but if anyone has to work on getting used to two or mostly more of these new aviation treats in too short a period, it leaves holes in the Swiss cheese.
Thorough assessment and/or adequate training should take care of that, unfortunately it does no longer due to economical constraints.