After seeing the magic that Rasmus has does with his side scrollers, I ALWAYS wonder if something "like" Commander Keen 5 stuffed into a FG99 is possible.
It's not magic, but a matter of exploiting similarities in the graphics and redefining characters accordingly. Unfortunately the more variation there is in the graphics the more characters it takes, and Commander Keen has far more detailed graphics than Bouncy's, which is already at the limit of what can be done with this technique.
Flying Shark uses another technique where characters are redefined on the fly. This allows for more detailed graphics at the expense of taking more time. But when we move from scrolling in one direction to scrolling both vertically and horizontally we basically need the square of the number of characters, and none of these techniques are any good.
So with the stock VDP I think the only option is to scroll in 8 pixel increments. Even then there seems to be far more graphics in a level than what would fit into a 256 character set. See here, for example:
With some clever programming it might be possible only to upload the characters you need for each scroll position, but it's far from trivial. There is also the matter of moving sprites behind characters which is quite difficult on a stock VDP.
The F18A is able to do the smooth scrolling, and its 8 color mode is also a much better match for the EGA graphics in Commander Keen than the stock VDP bitmap colors. It also allows sprites to move behind characters. But even with the F18A we have the limitation of 256 characters.
Perhaps if and when the F18A MK2 is released it will allow larger characters sets (a 16-bit name table)?
This analysis is only about the graphics. There are lots of other obstacles to overcome in order to port Commander Keen to the TI-99/4A.
Having said this, a game similar to Commander Keen but with simpler graphics is certainly a possibility. Then you would also avoid any copyright issues. Commander Keen has been released for the Nintendo Switch so it's not abandonware.