If he had of taken the role (which I'm sure he would have been very good at) he (as the captain) would have been kept in check by a loving and devoted wife.
I'm pretty sure that Jonathan Harris would have been good in the medical unit - perhaps developing a love hate relationship with the ship's computer.
Billy Mumy could have been a Wesley type character. Marta Kristen would have added glamour to the communications post.
Mark Goddard could have been the dashing first officer (in a type of Kirk role) while Angela Cartwright could have been the inexperienced trainee always getting into trouble on planets.

No 1 c/d rock Kirk like Shatner!

Poor choice of words on my part. Yes his own starship, not the "Enterprise". I agree with the other poster who said that everyone else would've had more screen time. I don't think Williams would've been as good a fighter as Shatner was as Captain Kirk though.

I always liked Guy Williams; "Zorro", the Lone Ranger, and Superman were my Big Three TV heroes when I was a kid (born 1955).
I agree that Williams would have made a fine Starfleet captain, but like a few others here I'd prefer he got his own ship.
It may not meet Vulcan logic standards, but I feel that, given Williams' presence and demeanor, on the Enterprise he'd be more of a "Number One"/Jonathan Frakes type character.
As captain of the USS "de la Vega", though, he'd be his own man.

Yes, definitely a captain in Star Fleet. As others have said, I can't see him as captain of the Enterprise but of some other starship. He would have been great!

There are likely other shows which changed their title music.
Happy Days
is the obvious example, "Rock Around the Clock" to "Happy Days."
F Troop
dropped the lyrics for Season Two, mostly because the sequence was in Black & White, and the instrumental track was re-recorded and jazzed up.
My memory is that
MAS*H
re-recorded the theme several times, including a later season bluesy version that was quite different.
Monk
is another radical change, moving from a nice, jazzy instrumental to Randy Newman's "It's a Jungle Out There."
Subtle changes include the
Mary Tyler Moore
show (which re-recorded the theme with different lyrics) and
The Brady Bunch
(which re-recorded the theme with the kids singing after season one) although these re-recordings of effectively the same song may be outside the idea of the original message.

The theme to
Gilligan's Island
was also retooled/rewritten after the first season as well.

Bonanza is the only other show I'm aware of that changed its title music, late in its run.
The Partridge Family changed its title music during S1. They went from one crappy faux pop song to another crappy faux pop song.
Dream of Jeannie had generic-sounding title music in S1, then from S2 onward it had that jazzy Hugo Montenegro theme.
There are likely other shows which changed their title music.
This is my signature line:
http://flapdoodlefiles.blogspot.com/

Duh. How stupid/forgetful can I be?
The first was by Bernard Herrmann, and (for me) it's the greatest title music ever written, or likely to ever
be
written. (It's sort of
faux
-Debussy.) It "hangs" at the end, not "resolving" until the end of the end-title treatment. Brilliant, brilliant, brilliant. "Too good for television", as they say.
The second is based on two short themes written by Marius Constant. It's certainly appropriate, but it's noisy and obvious, lacking Herrmann's atmospheric subtlety.

The Twilight Zone
used two main title sequences as well: This for Season 1 (
and
for the remainder of the series.

This message has been deleted.

You actually mean it had two main-title themes. (Unless they had lyrics, they're not songs.)
I remember the change, between first and second seasons, I believe.
Bonanza
is the only other show I'm aware of that changed its title music, late in its run.

Actually Lost In Space had TWO theme songs. As discussed by humorist/columnist/etc James Lileks in that podcast I've mentioned before.
https://ricochet.com/series/the-ramble/feed/
LIS is discussed in the second "episode," but the first should be heard anyway because the Walking Dead discussion is fun.

Yes, Williams would have made a fine starship captain.
Fifty years ago, I enjoyed the campiness of
Lost in Space
. Today, I can appreciate the occasional honest emotional moments. But overall, it's pretty bad. One wonders why they didn't' just shove Dr Smith out the airlock.
By the way, dinosaurs still roam the planet. They're called "birds".

back when dinosaurs roamed the plane,
They still do. They are called the GOP.

When I was a kid, back when dinosaurs roamed the plane, I loved Lost In Space. Now I can't even stomach the show. It doesn't wear well like Star Trek. On a positive side LIS, has a really cool theme song.
Back to the topic, I think Guy Williams would be a solid captain. Plus Guy was a team player, so the rest of the cast would have more to do. It definitely been a different type of show, probably more like Next Generation.
Spenser with an "S", like the poet.

Not as Captain of the E, that's Kirk's job, but, as a generic starship Captain, absolutely. He was a big guy and had a quiet authority, very athletic. The shift on
Lost in Space
from the planned family dynamic to
The Adventures of Dr Smith
was, on the whole, unfortunate.

I agee totally. He'd have made a great starship captain.
Duncan Idaho! Nice to meet you! Love your potatoes,man!