Is there any particular reason you think Margaret of Scotland would be more likely as a bride for Gloucester?
Once Edward had full right to do whatever he wishes with the lands confiscated to the traitors Neville (read BOTH Warwick and Montagu are attainted) then he has enough Northern lands to give to Richard, together with Middleham, to install him in the North and no need to marry him off to an heiress so the Scottish match would be more useful to keep good relations with Scotalnd and hopefully prevent borders skirmish…
Mary of Burgundy, naturally, is another very good choice but her match to Richard is unlikely to be seriously planned before Nicholas of Anjou’s death in 1473
Cecily Bonville is only 10 years in 1470 and her availability will depend much from Anne Holland’s fate as she was likely destined to marry Dorset as soon he was widowed (so I can see Cecily marrying Richard of Gloucester only if Anne Holland lived and she and Dorset had children)
Fair enough, that would make some sense. I do wonder if anyone in Scotland will take issue with the heir to the throne and the king's sister both having English spouses.
No reason for that as foreign matches for them are the likeliest option and using them for trying to keep peace on the borders is a good option. Scotland had already many English Queen (the last of them being James II’s mother) and princess Margaret had not much better prospects around (plus she has four siblings ahead of her in the succession)