Right on start take my honest apology for bad English. I'll use pictures to (slightly) avoid my lack of good english but at the same time it will be far easy to understand. Users which don't use outboard equipment won't find anything interesting in this long text.
I've created series of pictures on how some of your competition developed solution for using outboard FX. But first let's talk bout current situation in FL Studio. Can i use outboard gear? Yes but it's a hassle (really it is try it) to do it? Routing, two mixer channels, trying to sync everything is major pain especially if one want to use chain of effects or combination of Software and hardware effect in one channel. Then you are on your own in calculating sync. FL Studio is really bad in this area. Before someone ask why do i insist on using hardware equipment? It's because of sound which is different, and it is because of tools which does not have software replacement. Believe me i would me most happy person in the world for having everything saved in one FLP file. But that is still far from reality. Hardware effects such as Eventide Harmonizers, Roland Choruses, Bricasti reverbs and things like that, still does not have any software replacement but at the same time they are very very good at doing FX.
Same thing is for instrument. note that i am not questioning software instruments. Not at all. It's just that i still does not have any proper replacement for my D-550 (which is digital but there is no replacement still), My kawai k5000r is untouched but Harmless is getting close. JD-990 has some serious mojo because of it's FX. Sure its a digital VA but i couldn't replace it. I could go on with this. On the other hand software is getting better and better. I don't know for hardware which can do what Absynth, Harmless or Reaktor can do.
My point is that there is still need for hardware (i am sure it will stay like that) and users still use it extensively.
I'll show you how Cubase did it but in my testing i have found that majority of other competitive products are using same solution just wrapped in other skin.
Each screenshot has text which is explaining what Cubase is doing. In the end you'll see that i ended with sort of plugin which is created by Cubase and it is acting like plugin which can be loaded inside Cubase mixer slot.
What is advantage of this?
For one - Very convenient way on routing your outboard gear to be FX or Instrument. But most important - it will make everything in tight sync with other tracks. As you may know Cubase is calculating delay which is then aligned by Cubase PDC engine. Whatever i load before or after this, no matter which delay is added everything is in perfect sync with everything. Monitoring, bypassing everything is there and solution is perfect.
How this should/could be worked out for FL Studio?
Well i know for two ways. Ill explain each one.
first: Clone of Cubase. Why is this good? Because in Cubase you do this in global menu. Such as Fruity General option window. You set up everything from global option, and from there you go further. The good thing for this is that once when you set up your buses in Cubase in global option there is no chance that you can use channels which are assigned/dedicated to hardware effect for something other. There is no confusion. That said if something like this could be done for FL it could be done like this: You set up everything from Fruity general option window. there could be "OUTBOARD" area for this.
I mean routing, delay calculation and that's it. Once when this is done you'll have some kind of checkbox/swithc. Once when you enable this switch: fruity could generate some kind of FX in it's FX folder. This FX cold be named as your device (example). Obviously after plugin is enabled and generated by FL Studio, same mixer I/O channels which are used in configuration window - WON'T be available from now on. FL Mixer won't see it since they are assigned to our external FX plugin. So there is no room for confusion in available mixer hardware channels. That's it. There is no (imho) need for developing some special GUI for this. Just simple FL native gui with name of effect. Note that this effect should (obviously) report it's latency (which is set up in that OUTBOARD window i am talking about). Since FL handle PDC perfectly it should take this in to account. And that should be finished. Further development of this could add another neat feature which is: adding dashboards/midipanels in the same way Cubase is doing. That means one could have automation/banks/presets and midi panel via Dashboard feature. That dashboard thing is just is just an idea. Really just plugin itself will be enough.
second: IL create just simple External FX or External Instrument plugin. You load it inside FL Mixer slot and from drop down menu you choose inputs and outputs (which are available to FL mixer also) and you write down delay(calculated manually) inside that plugin and you are done. From this point I/O buses you chosed for plugin won't be available in FL mixer anymore so again there won't be room for confusion. And that's it. Seems simple yet very effective.
I am sure someone will argue to why do you need to add/set up delay value for External instruments? Shouldn't they be played/sequenced live: Answer is yes and no. Keep in mind that i can still use synth and patch it so signal is passing through various outboard gear which does include delay, therefore i'll need this to sync it up. Currently i have this setup here. I am using hardware synth which has slightly delay due to signal path. I don't use this for "live" playing i am sequencing everything from FL midi out
IMHO someone at IL should decide for FL to make it more hardware friendly. There is still ton of people using hardware. And there is still ton of people not satisfied with current support. You could attract even more customers by advert FL Studio as being "Hardware friendly". You already have best piano roll ever, sequencing option is stunning. You only need to develop these two FX helpers and make your won Dashboard more advanced. Not that other synthmaker dashboard. Your own Dashboard is quite amazing already. Give us sysex and you are done. We could have own forum area for this and slowly community could grow.
That should do it. I'll make copy paste from Cubase manual just for the sake of explaining things further in detail. In case someone at IL is interested to do this. Note that i will gladly and for free give my time to test something if needed.