Miriel, you have a point. Panne, however, well, Lucina has no visible signs of being a Taguel. Adding to that, Yarne's weapon knowledge upon getting him I virtually abysmal. So it makes sense why Panne can't marry Chrom. It probably would have been too lore-breaking.
As for my response to the topic itself...
Sumia X Virion, Stahl, Vaike, Kellam, Donnel, Lon'qu, Ricken, Gregor, and Libra - I can understand why Chrom's marriage options are limited, even though he himself could use some more options, namely Miriel and Tharja. Sumia, however, has no excuse. The player is literally limited to only 5 choices to marry her off to, and if any one of them, including Henry the obvious safety net in regards to this, ends up hitched with another girl, the player is left unable to get Cynthia. And there's such missed opportunity with some of these. Vaike, for example, could provide an explanation to how Cynthia's personality is what it is. Kinda like what Henry does with Owain, as far as paired endings are concerned. But alas. If the pairing limitations on Sumia is because of how the game "hints" towards Chrom/Sumia being "canon," then it just makes Chrom/Sumia come even more across as a case of being Strangled by the Red String. And based on some of Lazlow's dialogue in Fates, and how Sumia seems to be treated in Heroes, I think it's safe to say that I.S. realizes this.
Odin/Sakura, Oboro/Leo - I felt like the pairing options between Hoshido/Nohr pairings were a little lacking in options. And I always felt like I.S. could have put in more options. In the cases of pairs like Hayato/Selena, I think it's understandable. (That example probably wasn't included to help keep the identity of Cordelia's canon husband a secret.) But will others? I think there's some cause for inclusion. These two examples come to mind. With Odin finding use in Sakura in regards to providing Ophelia with another magic option. (Possibly with Odin's hypothetical Supports with Sakura referencing Lissa.) And with Oboro fitting the tailor theme Forrest seems to have. I'm not asking more much, though.
Chrom X Miriel and Tharja - Chrom needs more options. 'Nuff said.
And though these technically don't constitute as S Supports...
Rath X Any Woman Not Named Lyn - Unlike Sumia mentioned above, Rath does have somewhat of an excuse for why his supports are limited: Supports were limited anyways, Rath is a somewhat antisocial guy, and, if you really think about it, even with the fact that Lyn is the only playable character that Rath has a paired ending with in mind, nobody Rath can support with has any notable resemblance to Sue from The Binding Blade. But he needs more paired endings, even if only to put the notion of "Lyn/Rath is canon" or "marrying Lyn to anyone besides Rath causes a time paradox" to rest. It doesn't necessarily have to be someone in FE7. I'm sure a remake could include new characters like previous remakes did. Athena and Nagi in FE11, Katarina in FE12. Faye in FE15. One of the new characters inevitably introduced in an FE7 remake could be a Sacaean woman that comes from the tribe Rath and Guy belong to, the Kutolah. But if Rath were to be given options that already exist, they could capitalize on the fact that people whose nationalities are only half-Sacaean tend to have very little differences from a full-fledged Sacaean in regards to resemblance. Which could leave open possibilities to Supports and endings with someone like Farina, or Priscilla. (The latter of whom, I have heard has at least her ending with Guy end in her eloping with Guy in the original Japanese version. Can someone confirm this?) And it seems, as of late, that I.S. wants players to forget about them having given Rath and Lyn an ending. So giving him another female Support option, either way, would definitely help with this.
Hector/Fiora - Let's be honest. Hector needed more endings that made sense in context. They don't necessarily have to have be an implied couple like Hector/Lyn is. But definitely start the relationship at C and build with each Support. As opposed to feeling platonic at the A (like with Farina), or even just starting to get to know each other at the A Support (like with Florina). He could also use more options that don't have the player actually having to work for them to make them happen. (An average player can likely pair Hector with Lyn without much trouble, even if he or she is not intending to do so. But pairing Hector with Florina and getting them to A, though doable even in ranked runs, requires quite a bit of effort to make happen. Pairing Hector with Farina and getting them to A, however, won't happen if the player doesn't know what to do to make it happen.) The Supports themselves could also carry the familial focus that's only otherwise present in Hector's interactions with Lyn that occur outside of Supports, though not stress the importance as much.
Eliwood/Florina - This one was a little bit of a missed opportunity, in my opinion. Seeing as they're familiar with each other after Lyn's Story but before Florina joins alongside Lyn, Sain, Kent, and Wil. It could give Eliwood another non-Ninian option that would connect to the Marcus/Lilina B Support in FE6. And Florina's personality just seems to mesh better with Eliwood than it does with Hector.
Sain/Florina - This would have been more of an "opposites attract" couple than the male option Florina ended up getting. And could have provided some background on why Florina is so androphobic.
Rolf/Mist - Add another one to those who felt cheated by the fact that FE10 gave an ending to Boyd/Mist, but not Rolf/Mist. Some may argue that Boyd/Mist was being set up in FE9. But personally, I just don't see it. Rolf/Mist definitely seemed to have more of a thing going on in PoR. Even with ignoring the age difference between Boyd and Mist, the whole thing regarding Mist's pairs literally reminds me too much of what Toei did with Sora prior to the release of Digimon Adventure Tri. (Which is to say that the prospect of Sora ending up with Tai was hinted at in Adventure 01, and subsequently thrown out the window for seemingly no rhyme or reason in Adventure 02. To my recollection, even the original Japanese voice actors were confused by this.)