Most of the comments about WASD keys being softer compared to others I somewhat quite agree. The keyboard would probably then be too heavily biased to maybe FPS style gaming where WASD are used the most. Certainly the idea of mixed MX switches are not new as it has been pointed out above in both commercial as well as DIY sort of form, it certainly would be an interesting idea if such ideas were properly executed in a commercial sort of form. That said such idea is easier said than done.
I cannot stop thinking the idea of mimicking something similar to what Realforce did with their mixed rubber cups as variable weights. The idea if mimicked to work in similar sort of fashion on MX boards would be interesting. Though not really suited for gaming I guess the idea in general but I guess it is there to maybe offer better typing sort of experience for those who needs it.
If using my sort of idea, the idea is not without flaws. Certain MX switches I have noticed such as MX blues seems to sort of punish one if they tried to bottom out so in the respect MX blues may seem weird if used amongst others on the same set of keyboard. With a staggered sort of keyboard layout it maybe easier to implement various weights across each group of keys that the fingers may strike.
Another sort of idea comes to mind is that if the idea is to use mixed switches across various groups that the fingers are in charge of is to also furthermore extend the idea of ergonomics. I am sure if the idea is to minimise the strain across certain sets of fingers then the next step would be to maybe incorporate more ideas to prevent any more strain. Though ideally it is best to first see if there are any interest in mixed MX switches by "finger groups" first before adding ergonomically styled keyboards into the mixed MX switches range. Ultimately the idea may not be well suited for gamers for instance but art least this may give one the incentive to tap into another sort of area with those who uses keyboards mainly for typing but to also find something nice with both mixed Cherry MX switches (by finger group) and by the style of the keyboard that is there to further assist them in minimising strains if properly used. Split keyboard idea are not new, split keyboard with MX switches are hardly new, though split keyboard for instance with mixed and MX switches are new (especially if they are arranged in a specific way).
You could sort of say this is my bit of fascination with mixed Cherry MX switches on a keyboard being arranged as such to mimick variable Topre rubber cups across various sets of keys, but to combine that along with ergonomically styled keyboard would be even more interesting. I personally find the idea of restricting to just WASD key combos for varied MX switches too restrictive/specific. Not only would you entice a very small group of gamers (notably those playing FPS for instance or maybe even simulations) but again if such keyboard were to be used for normal typing for instance or even chatting within gaming, it would not be all that great. Then again to have more than just WASD using softer for instance switches along with few others would furthermore restrict the types of games and/or force end users themselves to bind any other game they may play to whatever keys that has the mixed switches that seems to be well suited for gaming. This in short wouldn't be all that nice.
Anyhow, this is just an idea that you could look at I suppose with a grain of salt. As for the idea of swapping keys out on the fly like LEGO style, maybe a modular keyboard with discrete keys and their switches?
I think that would not be a cheap option to be frank even though the idea is interesting.
