Your #1 is sound but the other 9 largely leave me somewhere between lukewarm and ice cold.
Design for 10 levels but leave it open-ended, with a great big caveat that going beyond 10th means you're on your own as the system may or may not continue to function properly, with the likelihood of functionality decreasing as the levels get higher.
And I agree with
@Gradine that there needs to be a lot of modularity - tweaks and dials to make the game more gritty or slower-advancing or more downtime-focused or more boardgame-tactical or whatever - but even with that I'm not sure your root proposals could produce a game I'd be willing to play or run.
Gradine's modularity was about class abilities/multiclassing. Modularity of abilities between classes so you can mix n' match to make your perfect class.
1. I like the 10 level structure.
but, since there is only 10 levels and full casters, add 10th level spells at 10th class level of full casters.
I guess I could? My intent was to give a scaling spell progression so at 2nd you have two first level and one second level spell. Then at 3 you get a 3rd level, and another second. Putting you at 2/2/1. And then just continue the 2/2/2/1 until 9th. Then at 10th level it's 2/2/2/2/2/2/2/2/2
18 total spell slots, evenly spread across all levels, and then do a reworking of the spell breakpoints and damage values to smooth the progression.
2. What about proficiency bonus?
2+1/2 level would be nice, +2 at 1st, ends at +7 at 10th level.
My plan was +2 at level 1, +3 at 3rd, +4 at 5th, +5 at 7th, and +6 at 9th.
3. Martials could do with more attacks.
more attack roll -> more fun
But also, more rolls --> more time.
Really just gotta find some kinda break point to ensure the fun without getting tedious.
maybe add +1 attack per 2 levels for best attack class(fighter), so extra attack at levels 2,4,6,8,10
barbarian, monk, warlord could be +1 attack per 2,5 levels, extra at levels 2,5,7,10
ranger, paladin,rogue,artificer could be +1 attack per 3 levels, extra at levels 3,6,9
If I were to do something like this, approaching it as clusters, I'd probably drop the lines you have here by 1, and push the bottom row to 1 every 5 levels.
But I really feel like approaching it individually would work better, since it takes into account the individual class' structure. Paladins and Rogues, for example, both function by adding additional damage dice to single powerful swings, while a Monk or Ranger should probably focus on taking more shots in general, with the monk stacking smaller hits while the Ranger focuses on several well placed shots. In contrast, the Barbarian's attacks should probably be less focused and broader swings that represent a more wild style.
By tailoring attack structures rather than going by broad categories you can get a more defined identity out of the class.
4. subclasses, if kept in similar format should start at 1st level IMHO.
features at levels 1,3,5&7.
Agreed, more or less.
I'd probably put a ribbon at level 1. Something to signify that you're part of this group, specifically, compared to other members of the same class without a big power at that point. Like giving Eldritch Knights the ability to use Cantrips at that level. Sure it's magic, but it deals no more damage than an attack and takes the same action. Theme and narrative establishment.
if power budget is watched, there is room for universal subclasses.
Absolutely is, yeah... well... kinda. If a subclass hands out Spell Lists for Fighters who have no spell slots it's still moot.
about TWF, don't know, maybe make light weapons really light(2 damage steps lower), d12->d8, d10->d6, d8->d4, d6->d3
and just have it that you make 2 attacks for every attack granted by your Attack action.
Could do something like that... but by your calculations a TWF Fighter at 10th level using a Rapier and Dagger would be throwing off 12 attack rolls every turn. Sure 6 of 1d8 half a dozen of 1d4, but the sheer quantity of rolling is probably too high.