The question for the cardinal electors will be basically what do they feel the Church needs at this time? When JPII was elected it was somebody youthful who could implement the council’s reforms. With Benedict, it was stability and continuity as well as a short papacy after JPII’s marathon stint. With Francis, it was somebody who could reach out to the world - the new evangelization. Beyond this, any serious contender needs to have leadership experiences, either in the senior ranks of the curia or heading a largish diocese or possibly an order.
What will be interesting at the next conclave is the diversity of the Cardinal electors. Some might make much of the number appointed by Francis but he himself was of course elected by JPII/Benedict appointed electors.
There are (at the time of writing) 122 electors, of which six will age out in the next 12 months and a further six 12 months after that and it’s safe to rule out a consistory happening this year. No non-Cardinal has been elected since 1378 and while, Francis’ election did being with it a number of firsts, there are good reasons why the election of a non-cardinal can realistically be ruled out - first, a lack of awareness of who are they are amongst the voting cardinals (Schneider may feature prominently on the pages of LifeSiteNews but I’m guessing Cardinal Mafi of Tonga for one isn’t all that familiar with him). Secondly, there’s the logistical problem of contacting them to get their assent to the election and arranging for them to be brought to the conclave without breaching secrecy.
When it comes to age, Clement X was the oldest at 79 and realistically that’s probably the upper limit although I think there’s an unwritten rule that anyone who’s retired from their position (as a bishop or head of a vatican office) isn’t considered papabille. At the other end of the scale, JPII was 58 but the election of a quinquagenarian (thanks Google) isn’t something we’re ever likely to see again. Admittedly, with Benedict breaking the retirement taboo, younger Popes may be more of a possibility - just not JPII young! So I’d probably say that between 65-75 is the most likely age. Old enough to be experienced but young enough to last!