In what way do you think the landscape of today's wwe would be different today?
Very different. For starters, Taker has been a mainstay on SD! after the brand extension and has filled in the role of legendary veteran. In order to make the brands seem more equal, i would imagine HBK would have had at least one year on SD!, and that HHH would have had more lenghty tenures on SD! than the year he had between '08-'09.
Secondly, a lot of guys that are viewed as 'top guys' would have lost the benefit of having a program with him. Think about it, after failing during his first WHC reign in '04, it took a feud with Taker to eventually help elevate Orton back up to where he was. After facing him at WM 21 and then during the Summer/Autumn of '05, Orton gained back some momentum and managed to get him self involved in a WHC title match at the following year's Wrestlemania. This could be extended to Batista, who, after injury appeared to have lost steam, not only had a criminally under-rated match at WM 23 with Taker, but a series of hard and brutal matches throughout '07.
Thirdly, i think Wrestlemania would lose some grandeur because Taker's streak is a big part of Wrestlemania today. Because the streak is nearing 20-0, his next Mania match is always interesting because we all know (realistically) that he will get there regardless of the opponent. Take this year's Mania. HBK-Taker II was last on the card because they were so intrigued as to whether the streak would be broken. Yes, you could argue that they also wanted to see if HBK would retire, but HBK-Flair wasn't last on the card at WM 24 so i'm assuming the only varying factor is that Taker was involved.
would he still be considered a legend?
Yes, i would say so. His 'American Badass/Big Evil' gimmick is usually disregarded by WWE TV as they really want to hammer home that he has been this undead character forever, so by not coming back as 'The American Badass' would not be a huge loss. His streak victims included Jimmy Snuka, Jake Roberts, Diesel, Sid and Kane, finishing up on 8-0 which is nothing to turn your nose up at. Feuds with Warrior, Savage and Hogan during his first year in WWE led to his first WWE title, albeit for 6 days. The creation of a huge stable like 'The Ministry' and he helped create the Hell in a Cell match which is so popular today, a PPV is dedicated to the match. So overall, yes he would still be classed as a legend but not as big as he will be after his current stint ends.