The S5 won't see a massive amount of non-stock roms, and certainly less than the Snapdragon S4. But there will be some with the architecture being open source unlike Somsung's Exynos. Also, there will be a lot of roms based on stock.
To be honest, I'm probably going to be happy sticking with stock anyway as certain features will be lost as soon as Samsung's framework is taken out of the equation (fingerprint recognition being the main one).
What I would really love to see more than anything else is an exploit-based root solution that doesn't trip the knox flag. But I believe the Note 3 4.4.2 roms still haven't had that so I'm not holding my breath for the S5. I'm still deciding if I should leave my phone unrooted when I get it on Friday or just bite the bullet and trip the flag the CF Auto Root. The idea of not being able to use Viper4Android isn't one that appeals.
Previous phones - Philips Diga, Nokia 6110, Nokia 3210, Nokia 7110, Nokia 8890, Nokia 9110i, Nokia 8310, Ericsson T28i, Nokia 6310, SE T68i, SE T610, Motorola V600, Nokia 6230, Samsung D500, SE K750, Motorola V3, Samsung D600, Nokia N73, HTC Artemis, LG KU990, Nokia N95 8gb, HTC Leo, Samsung Galaxy SII.