Serious Sam was never about being the prettiest. We loved the first two because of the game play. You ran, you killed, and you either kept killing at a brutal pace or you were overwhelmed by the insane number of enemies. That was it. No complex puzzles. No crazy strategy. Just run and gun and have a blast. Nobody cared that the lawn was flat because we knew if there was a lot of detail, the game would never have run at all. Way too many enemies on the screen to have serious levels of detail.
Now, many years later, computers are faster, we have more memory and all that, but the point of the game hasn't changed. With SS2 they changed things and nobody liked it. For me the main problem was it locked up a lot. I didn't have that problem at all with the first two. I still have the first two and I'd much rather play them as is than prettier versions of the same thing.
Why not instead take the original engine and make a Third Encounter? And then a Fourth Encounter? We LOVED the first two and two more games just like those, different maps and what have you, but same exact style of game play, would be awesome!