Should I wait or buy one now?
Buy what now? An Intel-based iMac now? An Apple Silicon iMac now if they announce that tomorrow (which I also doubt they'll be announcing)?
Nobody here know what's being announced tomorrow, so we can't tell you whether to buy it or wait.
IF they announced an Apple Silicon-based iMac tomorrow, we still don't know its configuration today. Would it have the same M1 SOCs with integrated graphics that are being used in the MBAs, MBPs, and mini, or might it have a different M-series processor with more cores, and discrete graphics? Without knowing that, and without knowing what your specific needs are, how can anyone make a recommendation?
From my viewpoint, if an Apple Silicon iMac was announced tomorrow and it had typical mid-range iMac specifications, I would likely get it now rather than wait for the next generation of iMac with Apple Silicon - I have a 27-inch late 2013 with an i5, 16 GB RAM, and Fusion Drive. It's still enough for my needs, but I'd like to replace it within the next year. Give me an iMac with a 5K display, all-Flash storage, and one of the current-generation M1 SOCs and I'd be fine.
But different people have different needs.