You have no data. You can't argue anything. (If you get data, you can, but you can't try to figure out what you're arguing before you have the data.)
Also, metathesis is a relatively sporadic ('irregular' across words) kind of sound change. There are very few instances (vowel+R clusters in a few languages maybe English?) where this occurs, but this is a historical change, not something you'd be modelling with OT (at least not in its most common functions). And if it's sporadic, it wouldn't fit OT well anyway.
Panini, as Copernicus said, aks is more common in Old English, I believe, and it has switched back and forth a lot. Whether constant instability (and free variation?) is metathesis or not depends on your technical definition, but it's an interesting case. Doesn't seem specifically relevant to this context though.
English is not a relevant test case because it doesn't have any sort of active phonological metathesis either.
So, you can decide between these options:
1. Find some great data.
2. Pick a new language.
3. Pick a new question.
Something somewhat relevant might be epenthesis. It's a little more regular than metathesis (though still can be sporadic). For example:
pumpkin and hampster
Which one actually has a P in it?
Or scents vs sense -- they sound the same.
You could try to look at some of those effects. But having a VERY specific question like this with no data is a good way to get null results.
The best thing to do is observe an interesting puzzle in the language and try to solve it, if possible.
Edit: just saw your reply. Aks/ask is complicated
