There's a demand for a larger 26" wheel hardcore frame with plenty of TT clearance
You're 6'5" - there is plenty of top tube clearance.
But you're half right - I'm looking at various methods of increasing standover for the new line of 160mm travel hardtails we'll be able to offer in the next year. We're really excited abut them, which have only been possibledue to big investments in tooling and lots of testing (a problem caused by CEN standards has led to us learning so much about how things work, we've made big leaps forward).
So anyhow - the longer fork lifts the head tube more, though to be honest, I've only really percieved it as a really a problem for smaller riders.
Though I guess you might wear baggy shorts and have very large testicles.
Anyhow.
Let's have a look at some numbers.
According to my Bikecad here Blue Pig frame, with a 140mm fork topped out gives a standover of the following.
20in - 32.9in standover (120mm head tube)
18in - 31.9in standover (115mm head tube)
16in - 31.0in standover (115mm head tube)
So - as you might expect I guess when you measure something in the middle, and keep one end still, and move the other end by 2in - things move up about an inch per frame size.
And if you compare the inside leg measurements of riders suited to 16in frames, to the inside leg measurement of riders suited to 20in frames (or your own inside leg - is it more than three feet??) - you'll see why I don't think you need more standover.
And that of course, is measuring standover in the middle of the top tube. Not nearer the front, which is where problems happen, if you see what I mean. And by shortening the head tube, to get the standover lower by lowering where the tubes start (a more effective way in reducing standover), then you get people using ludicrous amounts of spacers and wacky stems to get the bars up to saddle height, or above, in some mistaken belief that that's where they need to be).
The fact is, my 6'5" friend, very big bikes with 26in wheels look shit. I think 5'10" is the perfect height to have a cool looking bike.
And I bet you were that tall when you were 12.
Many "extended seat tube" bikes I've seen are actually just that - bikes with extended seat tubes. Not dropped top tubes.
They use extra supports to support the seat post so that it has less extension - still requiring seat post penetration to a half inch or so below the "true" top tube. All this succeeds in doing is stiffening the bike, so it has less compliance. That's not much fun. You'd be better off simply running a long post and a smaller frame without the reinforcements.