Thank you for your input, and I'd love to incorporate these all into the comments. I'll freely admit that certain classes haven't seen much play in my groups, making most of my decisions based purely on theory crafting, monk being one of them. But let's dissect your creative, fun, and MAD monk ideas.
Monk/Wizard: I greatly refute that wizard is the best class for a gish. IMO it will always be sorcerer with access to pretty much all the same spells, but the ability to swing and still buff in the same round with quicken, or twin things like haste. Bladesinger is a good way to add AC to anything, but is it worth the MADness? The loss of con really hurts a d8 hit die class that tends to be in melee, and +2 AC isn't anything to write home about. You are also doubling up on extra attack. The headband of int can't be considered as any build could be viable if a character had a bunch of 19s. What you end up with is an AC that is comparable to any other front liner, when bladesinging, and a still low AC when not. As long as this character is boosting dex his AC is the same as a standard wizard with mage armor on. I'm sure the build operates just fine, especially with an item giving a free 19, but it is definitely niche.
Monk/Warlock: For starters I'd probably never leave monk just two levels shy of Diamond Soul. You mention wanting to pick up resilient con, why not just take diamond soul and get all saves? Admittedly it does come rather late. As mentioned in the guide the Monk, doesn't do the darkness/Devil's Sight combo any better than any other class. It still takes an action to work, still doesn't always work, and still can alienate the team. The fact that advantage and disadvantage cancel out is moot point if your DM adjudicates that you can't attack what you can't see. Your party should essentially be picking a 5' square they think the foe is in based likely on perception, and attempt to attack there. A foe who does any movement should be very hard to pinpoint in the fray of battle with total blindness. In the meantime you'll have less AC and hit points than a standard monk, and until you pickup warlock, no benefit. The imp thing is a good idea. This does work, I just don't think its optimal.
Monk/Pal/Sorc: This one seems to just be a sorcerer/paladin that threw monk in because the player liked monk.
I mean the competition for bonus action gets pretty heavy with all the monk's existing options and now quicken spells. Quickened and twinned GFB/BB are great on a sorcerer paladin as ways to get a bonus action attack, and a way to imitate extra attack on separate foes. On a monk the, quickening a GFB will often deal less damage than using flurry. The only thing I see this build offering is a bit more mobility, and stunning strikes with a low DC. AC and hit points are lower than a paladin/sorcerer. It lacks the amazing saves of aura of protection, and has some very difficult progression between the 3 classes. It could be viable, but definitely a niche build.
Unless the community disagrees with me, I don't think these call for any changing colors. But please keep the ideas flowing.