My group should be doing a 5e game starting up somewhat soon and we’ve made characters for it, so I’ve been digging into it a bit more and hope to have a better feel for it in play in a few months. From what I have seen so far of 5e the above should work well enough for just replacing the usual actions (Action, Movement, Bonus Actions) with Martial Pool (although the base rules effectively give you a bit more flexibility up front simply because you have more available to you than a single MP die will give you at 1st level in Codex Martialis, FWIW). That being said, I’ll have to dig into it a little but but MP would need to be based on something other than Base Attack Bonus, since that doesn’t really exist in 5e.
Most everything is rolled as a straight d20+Ability Modifier (a bonus or penalty based on the Ability in question, such as Strength or Dex). If you are also Proficient in a skill being used for the roll you also get to add your Proficiency Bonus, which starts at 2 and increases by 1 every 4 levels, no matter class or anything else (even multi-classing, which in many ways is also simpler and easier to do in 5e).
So arguably MP could be based on Proficiency Bonus (either equal to it or more likely by subtracting 1 from up up to your MP limit of 4.) This would mean that MP is strictly a level-based mechanic and would seem to be the closest analogy to BaB in 5e, it’s just that class doesn’t affect it like it would in 3.5. That’s probably not a problem because, as we both know, people are in many way a lot more flexible and capable than previous editions of D&D tend to imply with a strict class system. This does make it a little easier to deal with leveling up a bit higher than, say, 6th level as you’ve suggested elsewhere as a method for tamping down the usual power curve (or overpower curve, as the case may be). In essence at equal MP would hit 4 at 9th level (and sticking at that ceiling at 13th level when Prof. Bonus would go to 5) or maxing at 13th (and not going higher at 17th) if making it at -1.
I think my personal take would be to make it equal to Prof. Bonus. That is 1 more die than normally available under the 3.5 version at the lowest levels, but it help to equalize it a little to the 5e rules in that it gives you a bit more flexibility in actions anyway than earlier rules did, but it will slow down MP gain a little (especially compared to, say, a 3.5e Fighter).
Feats are largely still there in 5e, although the “Feat” system changed a bit. There are class “Features” and you only get “Feats” (if using that “optional” system) when the class would get a Feature called “Ability Score Improvement”, which is around every 4 levels (4, 8, 12, etc.). You may choose a Feat instead of improving an Attribute by 1 point. The rest of the “Features” are otherwise kind of set in stone, although some Classes have a little flexibility with an “Archetype” you choose at 3rd level which gives you a branch of Features you’d be getting in the future. So some Features all members of a given class will get at certain levels, whereas an Archetype patch will give different Features based on the Archetype at some levels.
My take here is really the same as the BaB note for above, since CM simply adds “Martial Feats” as an addition to the standard Feats system in 3.5 linking them to BaB, although in this case they could be linked either to Proficiency Bonus (take one Martial Feat for Prof. Bonus, for example, which starts you with 2 instead of one, or Prof. Bonus-1 starting you with just 1 and getting a new one every 4 levels) or linking to the Feats system (you may also take a Martial Feat in addition to the usual “Ability Score Improvement” or Feat). The latter method would mean you start with none (unless you added a rule to allow for 1 or 2 MFs to start with) but would also mean you get a MF 1 level sooner than the former method (at 4, 8, 12, etc. for Ability Score Improvement rather than 5, 9, 13, etc. for Prof. Bonus).
In essence probably the biggest change here is that Class in 5e is disconnected from what had been BaB, and BaB is used as a core point of reference in CM. The biggest “Class” this effects, then, is Fighter since they would gain a BaB at every level, so they would be getting a lot more MFs (and gain MP very, very quickly) compared to what they will get using CM in 5e when linking it to Proficiency Bonus or to when you get Feats. So I think that may have to be considered for what seems to play well when using CM with 5e. (Not to mention dealing with the Class “Features” to see what is appropriate for use with CM or if any should be modified in some way). Advanced MFs would need a modified BaB requirement as well, of course.
I will say that so far I’m liking what I see in 5e. In some ways it uses a bit of a mechanic not dissimilar to CM in the sense of loading the details onto the sheet before play so it’s just a quick check of what to add or subtract from a roll and comparing it to a target number. Any table lookups and such are done beforehand to just get that bonus to use. Situational modifiers to a die roll generally come down to a single mechanic of having “Advantage” or “Disadvantage” (and if you have both they cancel each other out so you just make a straight roll). Having either just means rolling 2d20 instead of 1d20 and taking either the higher (with Advantage) or lower (with Disadvantage) roll. In essence what you kind of do with saying you want to give yourself and advantage by rolling 2 dice from your MP for, say, attack or defense in CM, but this same mechanic could be used for situational modifiers to CM as well, such that you take an extra die when rolling with Advantage (already a CM mechanic, after all), it’s just if you want to use Disadvantage to, say, remove a die (or add a die if down to 1 and take the lower result) as a way to use the same mechanic in CM.
Outside of that some pre-requisites may need to change just because some reference regular Feats that aren’t as freely available (or might not even exist now) as they might have in 3.5e.
Those seem to be the main concerns of trying to use CM w/ 5e, I think, from going through things so far. Superno may need some tweaking as well, but I THINK maybe a lot of that will be less affected by 5e changes than CM itself would be.