It would most likely need Itch.io, EA and Ubisoft to produce working linux apps working on the steam deck. If this works in the distant future you would be able to add them as a non-steam game and perhaps even start games with proton layer via ea desktop / ubi connect etc
Well, you basically are possible to install Windows on the steam deck, since it only is a computer, valve even has drivers for windows online. You might even be possible to split the ssd for steam os and windows, but I heard sometimes that Windows messes up the bootloader and then steamos will not boot that easily again… But I never made any experiences with windows on deck. I guess you will lose some performance, since the processor in the ally is better, but it might be worth a shot. If you decide to do so, keep in mind, that you perhaps need a windows key for windows on the steam deck
P.S. there are plenty of guides to dualboot windows and steam os or to fully replace SteamOS with windows