I appreciate all of that, 2097, but I miss weapons having special features (besides reach and finesse), so I thought having it be a feat would bring some of those interesting features back for those who want them, but still having the baseline weapons be relatively simple. The Shove action in combat is great for bull-rushing and tripping, but (I think) there should be a way for specially trained folk to do special things. I also like that baseline characters can try (and succeed!) at all sorts of special maneuvers.
I'd make use of the Advantage/Disadvantage system FAR more than adding/subtracting bonuses....as the Adv/Dis system was designed to get away from that kind of "death by numbers" mentality.
In any 5e game I run, if someone wanted to try something specific with a weapon, I'd look at what they were trying to do and if it was an attempt to "negate" something in the game. In the flail example, well, flails were designed to get around shields. I'd rule that *anyone* could try and use it that way. I'd simply have them roll at Disadvantage. Now, for a Feat...I'd simply word it so that it got around the Disadvantage aspect of trying that "trick". Many other feats do it this way, why not continue the pattern? For example, Crossbow Expert allows you to shoot someone who is in melee with you using your regular attack...not at Disadvantage as everyone else would have to do. So, a "Flail Expert" might allow the user to "Attack around cover or a shield without rolling at Disadvantage". This doesn't "add" anything to the rules, it simply uses the rules that are already there in a slightly different way.
One thing I hope they (5e developers) continue to do is stick to their guns with regards to "bloating the system". Bloat is bad. Don't give things that "break" the Basic Rules, and don't simply tack on a bonus number. The numbers thing is a horrible way to go, because after a very short while, a munchkin player will be able to pick a class, race, feat and arch-type and by level 4 or 5 have something like "Attack: +19 to hit (+2, +4, +1, +1, +5, +2, +3, +1) because they took "the right class/race/feat combo". Better to have them with "Attack: +7", but have 3 special things they can do without Disadvantage (or that they can do with Advantage).
Paul L. Ming