Using today's technology, you could [re]design the U-2 using more "stealth" technology as well as using a more traditional tricycle undercarriage. Everything else on the aircraft is fine the way it is! Of course the use of more carbon fiber components for further weight reduction would be a good idea as well. For every pound of weight removed is a pound of weight of more sensors, or higher in altitude you could climb. Physiologically speaking the pilot would still need a Full-Pressure-Suit, in the event of a cabin depressurization. Do away with the pilot you say? Ask Northrop-Grumman how their RQ-4 Drone (that was supposed to replace the U-2) worked out?
There is a limit to the altitude you can climb while using a traditional "Air-Breathing" turbofan engine. The higher you climb the less air(oxygen) molecules there are to support combustion, unless you go super/hyper-sonic and use a RAM/SCRAM-jet engine, but that uses A LOT of fuel. Traditional Jet engines can only consume subsonic air (No shockwaves allowed in the inlet). To fly higher you must either add more wing [area] (for subsonic flight) or fly faster!
Any aircraft that is optimized for high-altitude(thin-air) flight will always be a "pig" to land in the thick air. The U-2 isn't so difficult to fly, it is VERY difficult to land! More Wing Area for higher altitude flight would make it a bigger "pig" to operate/land in the thicker air. The U-2 was designed to "Loiter" for many hours on end and flying supersonic uses up all the fuel too quickly (Ask a SR-71 driver).
So, your DIY U-2 needs to be designed to be more stealthy, using lighter/more-exotic materials, with a slightly larger wing [area] (for higher flight) and tricycle gear. The orignal prototype took 18 months to deliver to the CIA for a cost of \$ 1-million USD (\$11,163,059.70 USD today). So, your biggest challenge today is designing and building one for \$11,163,059.70 in today's money! Not gonna happen, so just keep using the ones that are already paid for with 1986 money (those are the most current U-2S').
PS...U-2s rarely ever carry a "wet-film" framing camera anymore. They are up to "modern" standards in avionics/navigation and sensors.