Two thoughts on Rowlf:
Yes he's returned very very slowly, not always among the cast, and when he is he typically doesn't say much. I think this is really just more of a side-problem of "The Muppet Show dilemma" without a regular-exposure project like The Muppet Show to help smooth it out; back during the Muppet Show (where Rowlf got his second chance of stardom), there was always the problem that Rowlf, while destined to be a big part of the show could never really be among the group of characters that were included in backstage plots due to the difficulties of Jim playing Kermit and Rowlf. So we almost never saw him really being a part of the main plotline, so this was made up for by giving him his chances to shine onstage - on the piano and in skits like Vet's Hospital and musical numbers. In season one, you can see him constantly being thrown in where they could use him...most episodes had Rowlf in there one way or another - and each subsequent season, you'd see Rowlf used less - not to where he ever faded as a prominant character, but still not so "priority". He had his place in the Muppet universe, and an important one...but it was hard for him to move his way up to the likes of Kermit/Piggy/Fozzie/Gonzo/Scooter due to the backstage issue. The movies were the best thing to happen to Rowlf as it was easier to weave him into the fabric of the cast...but even then there was still a "seperate but equal" dynamic - he's one of the last of the main cast for Kermit to meet on his journey in The Muppet Movie; he's among the huge group in the Happiness Hotel in GMC, and MTM pretty much by way of format was very Kermit and Piggy centric since it got the others out of the way through the middle of the movie with the exception of their Traveling Matt-esque postcard scenes. A new series would be a great way to get Rowlf back in the semi-forefront again...and really since the Kermit-and-Rowlf-done-by-same-performer is no longer an issue, he could have backstage plots now...i'm actually a bit disappointed that he really hasn't been included in any of the Muppets.com stuff; that would really be an ideal place to use him more (but at least he did get his youtube chance so he's not TOTALLY forgotten there).
My second thought is the voice thing. Bill does a great job and i realized how great is really was when i watched You and I and George from the first episode of The Muppet Show and realized how close this actually sounded to Bill's take on the character....the big problem is (as others have said) he sounds VERY close to Bobo (as well as to Dr. Teeth...which also sounds very much like Bobo.) It's here that i really think Bill could learn from Jim's experience ... Jim had the same problem playing both Rowlf and Dr. Teeth who had organically developed on their own from different projects and eventually became main supporting characters on the same show (even playing similar instruments!) with very much the same voice. So if you track and play very close attention to Rowlf and Dr. Teeth's voices over the years (each season on TMS and beyond), Jim would work hard at establishing what made the two characters different...Rowlf gained more of a "country dog" type accent and Dr Teeth had more of a hipster-ish element in his voice (he also pitched Teeth a little bit higher...though this wasn't always consistent). For Bill to fully succeed in his role of the keeper-of-the-flame for these charaters, he needs to really give the same amount of attention to the subtleties of each character as he has not two but THREE main characters with essentially the same voice. I'm actually a little disappointed that he hasn't done as well as he has everything in him to succeed in this - he's very good with accents in general so it actually irks me a little that given this is a strength of his that he hasn't used it enough to each character's favor...and he already has a lot of the preliminary work done for him by all of Jim's years of working on the same problem. If Bill (or anyone in touch with him) silently lurks or reads on this forum, i directly address this to you...you were well chosen as the recasts of these characters - i feel you were the perfect choice to do them and this is really an awesome responsibility. It's not one that should be taken lightly, lazily, or haphazzardly. You have all the tools in abundance within you - take some personal time to do voice exercises, workshops, journal-writing sessions, whatever it takes to really focus your thought process on what is it that makes Bobo, Dr. Teeth and Rowlf different characters both psychologically and vocally...spend hours listening to how Jim would differentiate Rowlf and Teeth. I know you can do this and do this well - but it has to be done and i'm sure you've already done some of this...but it needs to be done more especially now that we're on the verge of the Muppets' long-awaited full return. You are a multi-talented performer who has lots of experience in different crafts - singing, acting (with and without puppets), directing - you should have no problem getting to the point where you can get these characters down into thier own unique place...but the work needs to be done; not glossed over. Take the time; do the research - do all those monotonous exercises where you sing a musical scale as each character or read the same monologue as each character trying not to make them sound the same to where it becomes more automatic. Your fans and colleagues are counting on you! And always remember if sometimes the criticisms of your voicing these characters seem a bit harsh, it's only because we fans know that you're fully capable of brilliance but aren't reaching the level you're capable of. Let's rock these beloved characters, okay?