Don't tell that to UTD, GSM, or Toddy... They have all standardized their systems, all three completely different, but all three are standardized.
That said, sidemount right now is no less standardized than backmount. Sure there is a "DIR" backmount doubles, but BSAC doesn't follow it, and there are a whole slew of alternate systems for single tank backmount, etc. There is also nothing that says evolution isn't possible, i.e GUE going from their SCR to the JJ.
@Darghu it depends on what kind of style sidemount you want to be doing and why. You have to find out what you want out of sidemount and what you need it for.
Is it for cave diving? If so, which style of cave diving? Sump? Cold water? Mexico?
Is it for OW? Wreck penetration, recreational OW, deep?
Do you need twin tank, or can you get by with single tank?
Is it a lifestyle choice/requirement mitigated by an injury?
Do you have dive buddies already diving sidemount that are passionate about a system for good reason?
Do you have a tie to GUE that makes you want to dive with them *i.e. buddies that are invested in GUE or specific projects you want to join*, or do you want to look at them because of the quality of their instruction and instructors?
Guys like
@PfcAJ can better comment about GUE's current and future stances on sidemount. I know what they've looked at over the last few years, but have no idea where they're planning on going, if they're planning on going there. I can comment on specific sidemount stuff after you answer the questions above and may be able to help recommend an instructor or at least explain the pros and cons of some options that you can look at based on your area, but without those questions we are stabbing at ghosts