Well, initiative in general has some real problems when it's broken down into player turns where nobody is acting except that one character. This is true not just of SR but also D&D, and hell pretty much EVERY game ever... at least that I'm familiar with.
Situations like mexican standoffs or a hostage taker holding a gun to a hostage's head are not well simulated via initiative. Another example is jousting. Or, in a more SR-worthy context, combat biking. Consider some runners and go-gangers engaging in some motorcycle-mounted melee combat. What SHOULD be happening is both bikers are approaching each other, then they both swing whilst in melee reach of each other, then (assuming neither is eliminated) they both wheel around for another go. You can't even MODEL that in "A goes, B goes" initiative rules where only one character's motorcycle is in motion during any given character turn.
So, the way SR has always dealt with this is to rely on GM "common sense" first and legislate second. 6th might rely on the GM moreso and legislate less so than prior editions, but it's the same paradigm as 5e and before.
So what do you do with the combat biking example? You might dispense with the movement and turn order, at least as the two jousters relate to each other. (gets more gnarly if these are just two combatants showing off during a larger battle) They might move simultaneously not via any RAW rule anywhere, but due to "common sense". You might or might not give the initiative winner the right to resolve that melee attack first, or maybe you make them resolve simultaneously. Maybe if the loser is using a weapon similar to a lance and has a substantial reach advantage, you let that melee attack resolve first no matter who won initiative. These are answers based on context rather than immutable rules. GM's perception of common sense is SUPPOSED to trump the RAW.
The example given a few posts upthread, where some NPCs tripped an alarm and the PCs want to turn the tables and ambush the would-be-ambushers? Sure. Again, a completely plausible scenario. But, the PCs are only screwed by the GM's lack of common sense, rather than the absence of an explicit rule for delaying actions. If the NPCs sneaking up on the safe house trigger the PC's silent alarm, the GM should be asking "what do you want to do about it" rather than saying "roll initiative". If the PCs want to rush to the windows and begin firing on the assailants, fine then go to initiative. OTOH, if the PCs want to wait and ambush the ambushers AFTER they breach the door, then the combat scene shouldn't even begin until the door is breached. NOW roll initiative, and if the PCs win, well there's the NPCs right there, all ready to be worked over.
Personally, I think judiciously determining when to "drop the time stop", to use an old MUSH term, is the key tool here. In the above case where NPCs triggered the PC's alarm, it's "wrong" to roll initiative immediately. At the point of the alarm, the PCs now have the upper hand. Combat shouldn't begin immediately unless THEY begin it immediately. If they want to wait for the NPCs to breach, then combat doesn't begin until after the NPCs breach. This seems obvious to me. In another example: holding someone at gunpoint. If A is holding B at gunpoint, warning him not to do anything stupid, etc etc etc then "that's not combat". That's a (tense) social roleplay scene. If B wants to rush A, that should be a decision made outside the meta-knowledge of who won initiative. Depending on contexts, as soon as B begins charging A shoots, and THEN you roll initiative, or perhaps if the context was such that B was close enough that it's plausible that B could reach A before A reacts, then roll init without giving A the "held shot" and let the dice determine whether A reacts in time. Again, GM's discretion is a thing so there's not necessarily any need to legislate whether holding someone at gunpoint allows you to shoot them "if they provoke you" during their action.
Now, while I think that MOST of the time there are better ways to resolve the concept of "held actions" than using turn order shifteroos, sure there are times when something like "spend a Minor to allow your Attack to be Anytime instead of Initiative" might be perfectly appropriate. Should that be a blanket rule you can do anytime without respect to specific contexts? I'm not sure, but again who knows what the upcoming combat book will institute.