Well I can’t imagine much of a difference, I mean look at Opposing Force and Blue Shift, they were all sort of the same: kill aliens, use the railway train thing, push buttons, solve puzzles… The aliens were a new in Opposing Force but other than that, the basic stuff is the same. So if I had to re-imagine Half-Life, I guess I would include and even larger variety of aliens (unlike HL2 which reduced the variety instead of expanding it), harder and more puzzles, more buttons to push and stuff to figure out…
I would also remove Xen entirely I think, or at least re-think it a LOT since in my opinion it was rushed and badly done, not organic enough and too repetitive. I would make it more magical and strange, with totally different physics and new elements and not just annoying trampolines… And less flying dickheads (Alien Controllers).
Nihilanth also sucked, I would make the final boss much more challenging with more thinking, using all that you have learned so far, and less random shooting.
I would get rid of the concept of Vortiguants being slaves and working in factories making Alien Grunts… That’s just kinda stupid, I liked the idea of Xen wildlife trying to kill you just for food. I liked the beast nature of the aliens and I couldn’t really fit the “civilised” alien concept into the game’s atmosphere.
Overall, I’d make the whole thing creepier, with some more cruel things going on deep down in the research labs, and I would emphasize more on the Questionable Ethics thing, with alien experiments, hybrids and stuff. I liked the Opposing Force Biodome Complex, I guess the two games could be combined into one, without the Black-Ops and the lame ending of Opposing Force (another quite lame and repetitive boss fight).
Half-Life is a great game, but after having played it 20 times I guess I see its flaws much more than I did before. Some parts are still awesome but I’m hoping that Black Mesa will once again give me that thrilling feeling that I had when playing it for the first time.