I intend to run an experimental play-by-post dungeoncrawling game where different player characters can be played with different rules sets. E.g. there can be a Pathfinder Gunslinger, Burning Wheel born peasant/hunter/trapper and OD&D magic-user adventuring side-by-side. Each character will interact with the world based on the rules that character happens to use. If there is PC-to-PC interaction, then we'll figure out something.
I will restrict the systems so that starting characters are somewhat on par with low-level D&D characters, so no Amberites or Nobilis Nobles get involved. I will also restrict to rules sets which give answers to questions such as "Does my character notice the ambush?", as opposed to "Do I get narrative rights to state that there is an ambush?", so no Universalis or such. Furthermore, no systems which require constant gm-player interaction during resolution will be allowed; e.g. Dogs in the Vineyard is not fine.
If the system uses experience points or similar, they will be granted on basis of treasure recovered (and maybe some minor factors), with the conversion rate determined on system-by-system basis. Another modification is that if the system allows characters to recover to full strength with great speed, this will be changed.
What problems am I likely to encounter in this experiment? How to get past them?
Here are two examples:
Initiative: There will be group initiative with all player characters acting first (or second, as the case may be) and all NPCs, monsters and environmental factors acting next, etc. This will break the scripted combat of Burning Wheel and probably some other intricate combat systems, but that's life.
Attacking an opponent: If several characters that use several different wounding rules attack a given opponent, what to do? This is still partially open, but converting hit points works for some systems and in others wounds cause penalties on actions - both of these are fine. More exotic systems might still cause problems.
Characters will not be balanced against each other and will advance at different rates.
Judging the difficulty of monsters and traps will be next to impossible.
Thus far there has been fairly little combat and also very few problems. I had to adjudicate N20 wound treatment on LotFP-powered character, and decided that temporary healing of 1 hp per level per 10 points in the result was the way to go. And the N20 character trying to scare a Pathfinder character was adjudicated as a command roll against DC similar to what intimidate would have had in PF, and the effects were as per intimidate. N20 and PF work on a reasonably similar scale, so this was not a problematic call.