Only Coby isn't Luffy's rival. He doesn't actually have a main rival. Roger had two in Garp and Whitebeard.
Here are the similarities between Roger and Krieg. You be the judge:
1. Both are huge men.
2. Krieg is the Strongest human in East Blue (Minus Luffy), Whitebeard is the strongest man in the Grand Line (World).
3. Both use a type of spear to fight.
4. As far as we've seen, neither have Devil Fruits.
5. Both have large crews with fleet commanders.
6. Both of their 2nd in commands fight with fire and their fists.
7. Both of their fist in commands are smaller guys. Though we haven't seen Marco fight, Gin fights with Iron balls.
8. Both want to find One Piece and be Pirate King.
9. Both are arrogant and prideful men.
10. Both have enormous ships as their flagships (Well, Krieg DID, rather).
11. Both had their dream ruined by a D.
Now, the last one isn't speculation on Krieg's part, but it may or may not be on Whitebeard's part. Personally, I think it's accurate. If Whitebeard had not been discouraged, it's likely he would have already found One Piece. Since Pirate King is apparently a postmortem title, he won't have that title until he dies.
To say Garp was Gol's rival but that Luffy has none seems sorta ridiculous. I can see Buggy, Ace, and Smoker all being rivals to Luffy. If Garp is rival to Gol, then Smoker is rival to Luffy. If Whitebeard and Gol are rivals then it's not hard to see Ace and Luffy as rivals either.
And some of your similarities are too vague.
1. Krieg wasn't that big. He was tall and muscular, I'll give you that, but to compare him being a bit large than the average man to a dude who's like 2 stories taller than an average man isn't very believable.
2. Comparing the East Blue, the tamest of the blues, to the most deadly sea ever is a like unbalanced as well.
3. A bit of a stretch since Krieg fought with a lot more than that spear and Whitebeard's weapon was more like a sword than anything, but I'll give you that.
4. True. Nothing wrong there.
5. Krieg did, but I won't argue that either.
6. That's not true. Pearl fought with shields. Just because he punched Sanji with a shield doesn't make him a fist fighter. He used the shields in ways that normal fists wouldn't hurt. If Pearl had just used gauntlets, then yeah, but Pearl was considered invincible because he was so well defended with SHIELDS. Also we haven't seen Ace fight with his fists to my knoweldge just because he punches to throw fire doesn't mean he's an expert fistfighter.
7. That's not really a similarity. That's like saying Samuel L. Jackson and Patrick Steward are the same because they're bald. Now the two could be compared for both playing famous sci-fi characters, but Marco and Gin have no similarities that we know of yet. Saying their body structure, which are far from extraordinary, is a connection is just trying to make vague connections where there's nothing there.
8. Fine there.
9. Haven't seen too much of Whitebeard to be sure, but fine.
10. I don't remember seeing any other ships of Whitebeard's left. Maybe he only uses one ship. But fine, big flagships.
11. Krieg's dream isn't ruined. He can still find One Piece. He lost a fight. Maybe his views of being a pirate are crushed, but it's not like Luffy broke all his bones so he could never fight again. If he did then Luffy ruined his dream, but as it is now, Krieg could beat Luffy, WB, and whomever, and become Pirate King.