There're very good in the comments: why would all games (except 1v1) rank the same: it's not the same defeating 11 players than defeating 3.
IMO the following measures should be taken:
1. All games are anonymous (prevents metagaming to some extent)
2. NMRs → replacement rule is default
3. You begin (new account) with zero points and can't lose points (unless a penalty for DCing/NMRing is implemented, but never for losing).
4. Winner gets as many points as factions are in the game: 7 in standard, 3 in Hundred, etc. Or maybe better make it x10 for easier fraction count: 70 in standard, etc.
5. Drawers get equal fraction of winning points divided times 2 (so drawing has a serious penalty). For example: a 3-way draw in standard gives 35
/ 3 = 11
(rounded down) for each drawing player.
6. PPSC survivors get the fraction of winning points without modifications (you joined a PPSC knowingly and some maps are so imbalanced that they require PPSC rules to compensate)
7. Scores get "rusty": on January 1 2015 (for example) the fraction of your score earned before Jan 1 2014 is divided by 2. Cummulatively it means that your last year's (and present year's) score is worth 100%, your score from 2 years ago only 50%, from 3 years ago 25%, etc. That way you can't live off past glories if you're not playing often (your score from 2010 is worth only 6%, for instance).