i've seen 7362 is being worked on (and 7490 up to a certain extent). Is there anyone willing to help to create an openwrt for 7590 as well?
The devices you've mentioned so far are all based on the lantiq vr9 (VRX268/ VRX288) SOC; all similar to some extent, which helps adding support for them one by one.
The AVM Fritz!Box 7590 however is based on a newer chipset generation (Intel/ Lantiq GRX550), which currently is not supported by OpenWrt yet. Yes, there are some attempts of working on supporting it, but they aren't really finished (nor merged into OpenWrt) yet - and support for the xDSL modem might take even longer than basic SOC support (DECT support isn't even available for the well-known vr9/ xrx200 generation yet). So if your intention is running OpenWrt on that device, that won't be possible now or 'soon'.
GRX550 + VRX518..
Maybe 7530 is an option. It uses a IPQ4019 and shares some components with 4040 which is supported:
openwrt cc is possible
modify uboot configuration, kernel need it,but dont flash it