Yeah, they could. The 77D does exist, so it's not like it hasn't been done before.
However, with next year being the Year of the Camera Body, and the RP and R getting spiritual replacements for cheaper end full frame bodies and at least one APS-C RF body, my bet would be on more of a "R90" rather than a 99D as the replacement.
I think both are equally likely to happen.
Canon undoubtedly have an APS-C camera in RF mount at some stage of development, and are just analysing demand before deciding whether to put it in production. I'd expect it to have a new sensor, maybe about 28-30MP, and to be styled very much like the R6. It would be the spiritual successor to the 7DMkii, but would have vastly superior AF, probably 30fps, and would be aimed at the birding and sports/action market. At least, that's what I think, and hope. The problems are that tooling up for production would be expensive, the sensor would probably not be used in other models, and if Canon's gamble didn't pay off, they'd lose money on it.
A "90D Mkii" or whatever it's called, is another matter. It would cost next to nothing to put it in production because almost all the development, tooling etc would be carried over from the 70D, 80D and 90D. Already paid for. A few minor changes, to spice it up just enough to ensure that it continued to sell. Instant profit with virtually no initial outlay.
Is there more demand for a "R7" or a "90D Mkii" ? I don't know. Is there in fact sufficient demand for either of them, let alone both? Again I don't know. Neither do Canon, it's a gamble. But Canon are Number One, so they can afford to take gambles. If Canon produced an "R7" and a "90D Mkii", my choice would be the R7, but a 90D Mkii would be much cheaper to produce and would probably help them to sell all the EF glass that they have stockpiled.