The side quest my group decided on was the Underdark adventure. They had five total rumors to choose from - diplomatic mission, trade caravan protection, backstory driven quest, local spy ring and weapons smuggling, or the Underdark - and the party decided to find out what the "rumblings in the Deep, something is moving in the dark..." is all about. Most of these rumors I basically picked what I thought was a neat bad guy or group of baddies to fight - a skull lord, a necromancer and a mess of shadows, a large pack of werewolves - and then created a rumor they could follow to get to said baddie(s). Really barebones stuff here, just a quick idea, a rumor to lead into it, and then flesh it out only after the party has chosen to follow that quest. What I have learned at this stage of the adventure creation is to look not only at the main baddie, but also who they are using as minions, who is a threat to the baddie in their natural habitat and how they interact, and any other interested parties that might be involved. If you are a player in my group, go away and come back in a couple of months after we have completed this side quest.
Gone? Good. So the "rumblings in the deep" is an aboleth, not the illithid like my party originally feared (still chose to go into the Underdark in the first place), and the aboleth is controlling a bunch of kuo toa (or they believe it is their new god), who are using quaggoth to wipe out a tribe of troglodytes who live close to where the party has discovered an entrance to the Underdark in a previous arc of this campaign, so that the kuo toa and by extension the aboleth have better access to the surface and potential mind slaves. The aboleth is a fantastic monster, and extra creepy due to the mind control powers, plus being nigh immortal, if the fight starts to tilt out of their favor, they just swim away and set up shop somewhere else. Also, they are not known for being big on physical belongings, and my party, having played in two campaign arcs I transferred over from 3.5e to 5e have more than enough goodies for their level. The kuo toa are another interesting choice, being weird god creators and also a monster which you can generally parley with as they are more interested in capturing you instead of outright killing you. Also, any changes to the aboleth's power block I can explain away as the effects of the kuo toa worshipping it. The quaggoth and the trogs are just your average Underdark grist monsters, they are there to kill or hide from or avoid or ignore as the party sees fit, and in any other setting would be a couple of other common monsters like goblins and orcs, or kobolds and bugbears, but they are Underdark dwellers so here they are. Pretty basic. However, looking into the kuo toa further thanks to the DungeonCast podcast (particularly this episode), I learned the kuo toa eat mainly underground dwelling fish and mushrooms. Who lives in the Underdark and are excellent mushroom farmers? Why myconids, of course, and I also learned that kuo toa are notorious slavers and lazy, so in the middle of the last session I decided that the kuo toa have captured some myconids (no sovereign) and the aboleth is slowly turning the whole clan into willing slaves for his kuo toa worshippers. This is a great opportunity for the players to do some role playing - do they save the mushroom-folk or slay them along with all of the other weird monsters down in the Deep? If they do spare them, do they negotiate a peace with them and ask them to move into the Shallows near their home area and begin trade with the surface? I also looked around the Monster Manual, wondering what could be a threat to an aboleth and came up with a purple worm - higher CR, bestial level intellect so immune to the kuo toa's and the aboleth's mind control abilities, and since it is mostly a mindless beast, it will strike at anyone be they in the party or not.
The aboleth was the seed. The kuo toa were the next step back towards and between the party and the big bad, and the quaggoth are crunchy pawns that the party has faced before, though at a much lower level so they felt threatened until the players realized how easily they were scything through them. The troglodytes were an issue, as they were just not relatable enough. You kind of felt sorry they were being wiped out by the quaggoth and the kuo toa, but then you remembered that no one wants a tribe of troglodytes near their cities and villages, and you stopped caring. But I could not replace them with myconids, too many moving parts to get the party into contact with them peaceably, all while the myconids are being wiped out by overwhelming forces if I had merely put them in the trogs' place in the game. So having the trogs as fodder no one cares about and setting up the myconids as prisoners/mind slaves that could later be rescued and dealt with diplomatically is the better way to go, after all. The purple worm is just a goad, an overwhelming force that if the party faces it head on will kill some if not all of them, so I can use it to guide the party around. A bit, a small bit, but a bit is better than nothing. The party had been very cautious approaching the kuo toa warren, and I needed something to force them to get there more directly - sneak in, bust in swords swinging, something - plus if the party is smart, they will figure a way to use the worm to their advantage, as I said before, the worm is a mindless beast and will attack anything it comes across. And it is always fun to, without warning, drop in such a large creature on the party and one they cannot simply punch to solve.
What was learned in all that was the addition of the myconids and the purple worm. The original seed for the adventure just had the troglodytes, the quaggoth, the kuo toa and the aboleth. But looking farther afield and adjusting the adventure mid-stream to include these other monsters from the environment really fleshed out this story, and will give my players a lot more opportunity to role play instead of just rolling dice and killing monsters. Yet, you have to be careful and not expand it too much. I definitely did not need to also include the drow, the duergar, the derro, Zuggtmoy, the mind flayers, and the githyanki/githzerai. Yes, they could have been added in, I could have led from this side quest into a major campaign arc that involved all of those, but since we are not headed in that direction, I left them out. Too much is too much, and while my original plan for this quest would have worked, it would not have worked quite as well, so I am glad I changed up my plans.
The other takeaway I got from trying out this rumor-based series of side quests and letting the party choose their path is that I realized that in addition to the multiple rumors, I needed to also decide what would be the consequence of not following each of those rumors. The consequences do not all need to be dire, or even considered detrimental, but something has to result from the party not chasing down that rumor. Remember, each of your rumors do not necessarily need to be mutually exclusive, maybe salt in a few smaller side quests that could be taken on, where the bigger ones do not allow any of the other side quests to even be attempted. For the consequences, these range from another local party takes them on and succeeds or fails, all the way up to something unpleasant has happened and now the party's next campaign arc is resolving that issue. For this stage in my current campaign, I am sticking with mostly minor consequences - the diplomatic meeting with the neighboring country almost ends in disaster but is saved by a rival adventuring party; the werewolf pack retreats away from civilization due to the party's next plot hook showing up; that strange town along the trade route goes oddly quiet as does the last trade caravan headed that way, and one of the PC's significant other, a member of the local militia, is sent to investigate along with their fellow troops; and the cultists do manage to capture one of the former party members for their grimoire (part of their background), but the local constabulary apprehend the would be kidnappers and everything is well. Of course, the consequences of not following these rumors is low because the rewards of almost all of these rumors is likewise low - a character level, an opportunity to do some character development, maybe a boost through some backstory relativity, and a chance to get in some role playing. With such low stakes and rewards, you do not want to run an entire campaign off of them, it would quickly grow too boring, but for quick, interlude type side quests, these lower reward/consequence side quests are perfect. And they are so easy to whip together, you could set up a table like a random encounter table, and roll to see what your rumors are, if you want to be truly lazy about it.
Which brings up another question to my mind - could you run an entire level one to twenty campaign solely off the rumors model? I am thinking yes, but if you want to truly run it that way and have an overarching plot that ties it all together, you will need to set up your decision trees beforehand. Just watch any of the making of documentaries on open world computer RPGs, you will get the gist of what is involved. And looking at a recent Wizards of the Coast campaign setting release, Eberron: Rising from the Last War, they have provided plenty of plot hooks tied to both organizations and geographical locations you can practically run your Eberron campaign from those plot hooks. They do not really provide any solid overarching plots, though, you will have to come up with that one on your own. Reminds me of what I have seen from computer RPGs, at least the "making of" documentaries about them.