I'm planning to buy this ASUS RT-AX88U router as a futureproof, ax solution. Is there any chance that OpenWRT community starts supporting this powerful device in the near future?
I know there are alternative firmwares, like Asuswrt-Merlin, that allows more customization than the stock firmware, but I would like to use OpenWRT ecosystem - if I can - in the future as well.
That's virtually impossible, it's an all Broadcom (BCM49408 SOC, BCM43684 WLAN) design, for which no wireless drivers exist.
If you're looking for 802.11ax devices with a reasonable expectation of future OpenWrt support, you should consider QCA-Atheros instead (partial ipq8074 SOC support has been mainline for around 2 years, ath11k wireless drivers will be part of kernel v5.6). That isn't a guarantee either, nor will they be supported immediately, but chances aren't too bad.