Sounds good to me! This has been my kick in A&A for years heheh. Even though it's possible to play effectively with very long logistics lines if you plan several turns in advance, and even if buying a new frontline factory somewhere doesn't really change the underlying dynamics with that (you'll still need to plan ahead and shuck from the core regardless in order to play well), I still think it helps a lot. It gives the player clear goals with clear payoffs, and presents them with thematic TT targets to go after. I think having spots like that is just good for the gameplay, since it's accelerates the pace, and rewards the player for doing what they want to do anyway, which is to shorten the lines and push their fronts.
For Neutrals, my strong preference is the standard TripleA attackable neutral with standing armies and low PU values as a deterrent. In the older TripleA games these would be shown as having a salmon/orange hue to distinguish them from the beige 'impassible' TTs of Revised.
There have been numerous attempts in A&A to conceive of Neutral TTs in various ways since Classic. Global probably has the most complex, settling on Pro Axis Neutral, Pro Allied Neutral and True Neutral, as well as the Dutch and Mongolians which have their own stuff going on. To me that's just more moving parts than are really necessary and I much prefer a scheme where the player only has to deal with 1 kind of Neutral, and of those the attackable Neutral seems the simplest.
Frostion opted for 3 kinds of Neutral, even though I suggested many times to ditch that as unnecessarily complex heheh. The main problem with allowing movement through Neutrals, is that you invariably encourage the player to start knocking them off in order to disrupt the opponent's movement. Or you'll have situations where the enemy can just camp out stacking units in places where they can't be reached (the Brit fighters in this case).
To my way of thinking, having Spain or Turkey, or Sweden or Switzerland routinely dragged into the fight, is even less plausible than the idea of war breaking out between Japan and the USSR before the ink dries on their non-aggression pact hehe.
The Spanish landing pad in Classic is a somewhat notorious example of what happens otherwise. In the Classic game any neutral could be occupied for a cost of just 3 bucks! lol
I don't like the pay-to-play on Neutral ground dynamic though. I think if you want to discourage players from attacking the Attackable Neutral TTs, the cost of doing so has to be pretty high. Meaning a low or no value tile, where the standing Neutral army can reliably put up some hits. So an indirect cost, in unit attrition, rather than a direct cost in PUs to invade. If the tile is particularly attractive for the movement logistics, like say Turkey controlling the straits into the Black Sea, this recommends an even larger standing army or defensive force to give the player pause before going on a Neutral crush.
I would distinguish between Neutrals that become Belligerents during the later course of the war, and Neutrals who remained neutral throughout. So for example, you might have a spot like Iran (or say Belgium or Poland if the start date was 1939) that was designed to be attacked, with a smaller defensive force or a higher value if it's conquered. Whereas a spot like Switzerland, that never joined the fray, might have an artificially larger force/lower TT value to reflect that. Basically taking some stuff off the table in practical terms, but without needing a hard prohibition. Just making it less attractive to pick off the little guys, unless it's thematic to do so, more or less.
ps. One last thought on Attackable Neutrals... So even though this is my preferred type of Neutral, there are still some challenges with it. The first is that, as the game goes on, the cost in unit attrition will diminish as the number of units in play increases over time. So it could very well be that after a few rounds, just a couple hits isn't enough of a deterrent anymore, and the player may not care how much the tile is worth, if the goal is simply to open up another attack lane vs the opponent. To me this would recommend a standing Neutral force that increases over time. Like add 1 inf per round, or something similar to the neutral stacks. Of course that would also incentivize the player to kill early if they're going to kill at all, but to me that is also thematic and fairly sensible from a gameplay standpoint. Like if it's going to be a game where Franco joins the Allies, or the Turks throw in with the Germans again (ie they get conquered by the enemy) having this crack off at the start of the war rather than as a feature of the endgame would probably make sense. This also allows for the possibility for a endgame Neutral crush, but ensures that whichever team attempts it, they'll still be some kind of offset to the challenge, when the players stacks otherwise would just dwarf the neutrals by that point. Another thing to consider, when neutral become attackable is to watch out for movement exploits via strafing/retreat. This is something that came into focus for me with the very first TripleA game I ever made that used attackable neutrals (which were then a relatively new feature.) So in the Great War game I made with Surtur, the Central Powers could attack into Romania with a strafe from multiple TTs, but then retreat the entire force to a single TT which allowed the units to move further than they'd have otherwise been allowed to move. So basically you want to watch out for Neutral TTs with multiple connections to make sure they don't allow for quick skips.
Also just to say, I understand the idea behind having Allies able to move through Neutrals, while Axis must conquer them, but gameplay-wise I think this can allow for strange stuff to happen. Like where one team (Axis) would avoid attacking a spot, to prevent the other guy (Allies) from taking direct control of it. In Frostion's this would be a situation where Axis does not want to conquer Pro Allied TTs, even if that might be thematic (say in N. Africa) because doing so would then allow Allies to claim the TT directly on counter occupation.
A related phenomenon in G40 would be like Allies trying to claim a spot but having it revert to another teammates control. Like Normandy going to France after being liberated from the Germans. Since France can't really afford to build production, but USA can, you end up in a situation where Allies might not want to Liberate Paris for the French just to hold onto the Normandy factory or whatever. Or vice versa, where Germany doesn't want to conquer Normandy initially, because it's easier to manage under the French opponent than risk the TT being occupied by UK/US who can use it more effectively. Gamey stuff that can be hard to predict, but which might shape the gameplay in odd directions. That's why I like the whole original owner neutral thing for some TTs, like where we know big D-Day landing and such might occur. Obviously there are no Frenchmen here, but you know, same thing can happen with any pair of teammates. That's another reason to recommend having more than 1 potential mini-hub per region, so the Brits can take one and the Americans the other, and without the move screwing over their buddy too hard. Or in a game where 1 of those nations is controlled by an AI, and the player is competing with their own teammate to get to the desirable lily pad production spots. Might also be a reason to have more than one spot. Often in A&A you'd have a dynamic where, if Allies were going to expand production in the Atlantic, that one guy would do it Scandinavia and the other in France, or something similar. But here there are enough TTs that you could do like Normandy and Belgium or whatever, so both guys got a spot they can take and for juicing the production spread. I like it, especially if Germany could potentially retake and drive them to the sea, which is usually the danger of expanding production in the smaller scale games. Like that you're just gifting it the opponent hehe. But that stuff is all in good fun!
In general I think it will make the game more exciting for all players, since more potential targets means more expansion routes. Especially in the Pacific, where what happens otherwise is that everything gets bypassed and players only focus on the potential production hubs in the Dutch East Indies. Having more spread around could only help with that I think, and help to define the Central Pac as it's own theater instead of just playing second fiddle to the big money islands, like always seems to happen in A&A hehehe