In Barry Allen's first appearance as The Flash in the comics, he's introduced reading a Jay Garrick Flash comic. The characters in the movies being shown in that theatre could have been the Superman and Batman of Earth-2.
Hulk want hug kitties
But they so easy to squish
Hulk live in cruel world

I did a quick check, Batman #258 (1974)

while 'Arkham' was name checked. if I remember my timelines, the Flash Episode actually Predates Arkham Asylum in the comics

The episode Child's Play had a scene where Barry walked his dog past a movie theater showing Batman and Superman as a double feature, which should establish that those characters are fictional in this Flash's universe. Though Arkham was also referenced as being the subject of an expos in the preceding scene, teasing us and then dashing our hopes.

hard to say, had Flash lasted Longer Maybe CBS would have picked up Lois and Clark too
I think Tonally Flash and L&C would have made the best match up all things considered

Except they were different production companies producing the series for Warner Brothers. Warner ran its operations as seperate entitties, which caused major marketing problems for years. For instance, even though Babylon 5 was distributed by Warner Television, the DC comics came about through meetings between DC and the production company, not through divisional meetings at Warner. Warner was never good about cross promotion.
I think guest appearances by other heroes would have been fine, in a second or third series, but not in the first. However, I suspect they would have been limited to lower eschelon characters, not the Trinity. A crossover with Jay Garrick would have been great, using the parallel world or alternate dimension premise.

another poster is correct. They probably would have tried to merge Lois and Clark and The flash into the same universe, with a couple crossoevers. That would have been sweet.

I would guess Hawkman, as there is a reference to "Professor Carter Hall" in one of the episodes.

If it weren't because Elongated Man could only be done as animation (which is still the case, CGI = animation).
There's no Santa Claus

Thing is though with the Flash, having the odd guest appearance by another hero would've fit as the Flash is a very central character of the DC Universe. The Flash always plays a pivotal role in every "Crisis" in the DC books and is in many ways, the heart and soul of the Justice League. Hell, if nothing else, they could've introduced Elongated Man as he was introduced in the Flash comics. Or maybe even one of the Green Lanterns as the friendships between Flash and Green Lantern throughout the ages is one of the strongest teams/bonds/friendships in comic history.

Actually, in a late-1960s issue of "Superboy," the boy of steel did meet Aquaboy; and in stories from the 1950s, he met teenage versions of Oliver Queen and Bruce Wayne.

I'd have preferred that they not do this. On Smallville, they have Clark meeting A quaboy(?), which Aquaman never was, & Green Arrow. Unfortunately, Oliver Queen didn't become Green arrow, until, as an adult, he was stranded on a desert island.
Carpe Noctem

Another fun fact Henry Cavill was also the second runner up behind Daniel Craig in Casino Royale, especially since it was going to be a reboot, but they decided to go with an older route instead of a younger one. And he was also Stephanie Meyer's pick for Edward in Twilight but when the movie started production he was fortunately already too old.

actually GC supposedly tried out for Lois and Clark and the Trivia on his page Claims he was on the short list, but was passed over when they realized he'd been superboy
Personally I think this Is just Him stroking his own ego trying to make himself look more important
Given the fact he'd played Clark/Superboy for 3 years and arguably those episodes would still have been airing during casting, I find it improbable that a casting Director would either forgive a 3 year Hole on his resume, or overlook his 'present job'
as to Superman Returns He'd have been way too old to even be considered seeing as it was 23 years after his Last Superboy episode
one interesting note on Returns however is that Henry Cavill, who assumes the Cape in 2013's Man of Steel, was on the short list for 06's Returns, but was passed over for being Too Young

I read that John Haymes Newton was let out of the show because he really couldnt act. I do recall seeing some of the show with Gerard Christopher and he was a good actor. I do have the first season of Superboy with John but have not watched it yet. I actually had never seen John's acting until I saw a movie that came out last yr called "The Christmas Card". He really is kind of devoid of charisma so I can see why they say he cant act. He has the looks but not the presence.
I also read that Gerard tried out for the Superman Returns movie and they were very interested in him until they found out he had played Superboy and they told him he has already played the role and they want someone new and unknown. Christopher would have done great with it.
A neat coincidence, I never watched Melrose Place but I heard that both John Haymes Newton and Gerard Christopher had played characters on the show at different times.

If it had gone into the same time Lois & Clark aired (1993-1997), Dean Cain probably could have/would have.
Oh, on the Superboy post, Gerard Christopher was the actor who played Superboy for the last 3 seasons, so it would have been him instead of John Haymes Newton.

By 1990, Superboy was portrayed by Gerard Christopher.
I really need to get my MySpace moving.
www.myspace.com/jillange

Like John Hayes Newton(sp?) who played Superboy in the series of the same name.
Bring back Scott & Charlene to Neighbours.

I could see them possibly bringing in other characters, but I highly doubt that Christopher Reeve would have played Superman. They most likely would have gotten a TV actor.