Whoopi, Robin & Billy Crystal were all dear friends, from back in the days of their "Comic Relief" homelessness charitable endeavor together. I actually think that if Billy had been the one to teach Data about humor, (Instead of Joe Piscopo) they might have been able to rework that awkward mess to actually be ok
Anyway... It surprises me to think that a star as huge as Williams was, at that time, would be interested in doing the show, but it IS Star Trek, & Whoopi's presense proves what an allure that held. The only reason I could think he'd want to do it was maybe he was a fan, like Whoopi, & the two had conversed about that interest... it got back to Berman, & they attempted a vehicle for him, which never panned out, because let's face it, it probably never WAS going to pan out. The man was Robin Williams. He might have even saw the script & just thought... "Meh... not worth returning to the small screen"
As to him playing Rassmussen, They literally would have to have let him run wild with it, because I don't think the story or writing is all that great, at least not at the level of talent Robin Williams was. So basically, it could've been brilliant... IF they just let Williams own the thing his way, & drop whatever construct that script had going on. Him playing things like what we saw Matt Frewer play? Waste of time for him.