So, here is the situation: I am DMing an RPG group that is almost finishing one of those old and small RPG books with introductory campaigns (written by Steve Jackson and NO GURPS!).
The problem IS: This group is my family, summing up 17 people. Not everyone appears all the sessions, but there are a few (3~7) that are mostly always present. Sometimes a random friend of someone appears and joins the game to play once or twice(I personally can handle this by turning him into an NPC until it goes away/dies). Most of them are NOT into numbers, and get confused easily with "complex" systems, or by the NEED to keep track of so many things at once(but not all of them, one or two can be the "banker" and still play normally). As a side note(not a problem itself): they can have any alignment, but none of them will ever want to be evil.
So, bullet points: - Flexibility in the number of people (single game handling from 5 to 17 people in a party, commonly 9~10) and their frequency/rotation on table.
I like the absurd freedom of choice of players("I want to hug that evil bad-looking zombie-demon and ask it to be my friend! :D" ), so choice flexibility for DM and player is also a must.
Most choices made should have some meaningful impact in the future, as I'm thinking in a sort of sand-box campaign with some hidden and bigger plot going on. The setting will be somewhere around medieval-fantasy (a deep and open campaign setting)
Being a party SO BIG, combat must be practical, no number crunching, fast and flexible (probably using the PCs aptitudes paired with dices, cards, w/e).
The system can have bookeping, but must be kept low OR optional , as only about 2 to 3 of my family members are capable of doing so decently. Something like one player keep track of the money for the whole party until shopping-time.
There must be an explicit progression system going on their characters, as well as the world around them (characters getting stronger and better, world getting somewhat more challenging where they are). (PC development)
There must be a good variety of choices, especially for classes/roles(no need for explicit rules, just have room for it). Some may want to be a wizard, other warriors, and one wants a pirate...(the PF gunslinger-fighter mix? xD)
greatly desired, but not a must, is the ability to reuse and adapt content from other systems(namely pathfinder, that I am playing as a druid in a party of 6).
I already looked upon some systems mentioned in similar questions, like Fate(very good on the practical combat, the fate tokens), but I suppose my group's crazy varying range of people playing and the sand-box nature of my campaign would conflict with the plot-based nature of Fate. Sorry for the length, but I thought in as many details I could for the question.