Well, let's see...
BOWSAH
What moves or tactics should we watch out for?
Bowser has some cool tricks under the shell, here's just a few that are most noticeable:
-Whirling Fortress: You know that annoying upB ooS option that Marth and MK have? Well, Bowser's got arguably one of the best, due to it's almost entirely safe nature and nasty damage it will do (it'll even kill at higher percents). This thing will be pulled out on many an occasion Bowser is planted firmly onto the ground, and if he's shielding, you better change your spacing while landing from the air, because crossups, fakeouts, all of them don't work very well on a bowser who knows how to flip it on and scuttle away. Luckily, Lucario doesn't have to approach, which helps reduce the impact of this move.
Flying Slam: Also known as the "klaw" etc., it is an aerial grab. It also has autocancel frames on most of it, and can be used to mix up his approach options. In a MU where there's a projectile involved though, some of those options tend to be limited, but you still have to watch out about getting too close to a bouncing boozer, some of his moves will surprise you. A habit that will change in this MU though is shielding aerial approaches. If you aren't fast on your feet with that shield and you don't retreat or do something, he will nab you out of your shield and make you eat 18% or so, which gets you that much closer to getting high in percentage.
-Grab: If Bowser gets a grab on you, you'll be in for some pain unless you jump break. For those who don't know, Bowser basically gets 10 or so frames of advantage out of a ground break, which means unless you're DK, you'll be eating either jab, Flying slam, ftilt, or dtilt, even another regrab if he feels he wants to risk another set. jab, flying slam, and ftilt will usually be used to get some fast damage, or get you off stage for some edgeguarding if he feels he can do stuff, while ground release -> dtilt is an insured killer against lucario in the 130's, even with good DI. Frankly, you'll want to avoid the grab, and if you do get grabbed, try for a jump break. Also, sometimes bowser will try to do ground release -> jab -> sideB, but this is escapable.
-fair: This move is pretty bad news for lucario in early percentages, and is another reason lucario SHOULD NOT APPROACH Bowser. This move actually has more range than marth's fair, except bowser's hurtbox gets extended. That being said, this move is going to make any offensive attempts go sour, because the trade off in percent is horrendous regardless of most of lucario's horizontally oriented aerials (3%-7% vs. 13% is awful, do the math). He'll also try this move for edge guarding, but it won't gimp kill until later percents.
Ftilt: Another defensive move, acts as a landing catcher for pretty much ANY landing within it's range. Be aware it can cancel AS, and it its range is pretty deceptive. Don't use your options predictably when bowser is in a situation to use this move, he'll pretty much win it every time. It's almost a frame trap in its own right.
Bair: This move will be more of a gimp move, if you see it coming, it's way easier to move away and dair stall and try moving back towards the stage than to try to trick it with AD or something, since he has to commit to this move for the most part.
Utilt: another one of bowser's moves ooS that works pretty well, it seems to pale compared to dair even though it has some good range, due to speed and dair's properties. Still a move to watch out for.
Uair: A "hypnokill" move, this move is a nasty early stock ender if you aren't thinking right. This move will only sometimes be use after upB connects or utilt, but if you see it, don't AD, try dair stalling or double jumping, it's windup is deceptive, and it will kill in the 60-70 range reliably if it hits. Be smart within the 2nd jump area above bowser's head.
flame: Mostly used for edgeguarding, this move will hurt if you let him set up for it, although AS will start going through it at relatively mid/high percents uncharged, so that's not too much to worry about. My friend doesn't use flame as much as some other bowsers, but it's going to be a good edgeguard tool vs. a recovery with a more predictable windup. Play smart, and if he does use this onstage (he will occasionally), you'll need to DI away from bowser to get out.
fsmash, usmash: It's surprising that these will work, I wouldn't worry too much if you are fast enough, but be aware that fsmash will sometimes be used as an option if you hang on the edge for too long (dumb player habit to begin with anyway), and you also will want to be aware that usmash has very low endlag, so act quick if you want to punish. They are deceptively easier to pull off than you would think so beware of what he can do with them.
, but I can say that Lucario can be problematic for bowser if used properly.