All the new mirrorless models have similar image quality with APSC DSLR.  You might as well look at the lenses of the systems to decide what brand you want to invest in. 

If you're already with Canon you can go for EOS-M.  It has the cheapest native UWA lens with the EF-M 11-22mm. 

m43 and Nex have native lenses but quite pricey.  Olympus 9-18, Sony 10-18, Panasonic 7-14.

Fujifilm is seen as a quality mirrorless but for the UWA it only have 14mm (21mm FF equivalent) which might not be wide enough to your liking. 

As for filters you better check if the lenses can take filters then find if it has an adapter to take it.  Some UWA lenses can't take any filters.

that is correct.. not many will take filters.. I have the SONY SEL1018 and it takes filters.. Most UWA adapter/ add on will not take filters.

If you like to play with long exposure then Variable ND will be part of your needs and you have to make sure it can take filters.  Initially, I thought I could live without the filter  option when I get the adapter.. It was a compromise in many ways..