Main issue I see with GOC is how foundation reliant it is, GOC essentially can't do anything without foundation letting them. All the RP on the server happens in the foundation, surface sucks and is only used for PVP between foundation and CI. As GOC is a neutral faction they inherently have nothing to do unless they are allowed to do it by either foundation or CI.
While im sure it isn't super popular I think the best way to enforce a larger GOC presence is by making them more combative. GOC and foundation have a similar goal with completely opposing methods of performing the goal. CI and GOC just have completely opposing goals and should only be working together in extremely rare scenarios.
As far as base changes go I think the only thing that would provide tangible benefits is a larger lab with a slightly better SCP containment area for testing, besides that the base is completely fine, a base rework wouldn't fix GOC's issues.
My suggestion can be broken down into 4 parts.
More suits
This just makes the GOC a larger threat combatively that needs to be taken seriously, these suits should be something that needs to be obtained through gameplay. I will go into more detail below but some of these could be obtained after raiding a specific warehouse, ADBing an SCP, raiding a location within the foundation for supplies.
Give GOC the ability to be an enemy of the foundation and CI by default.
This is obviously the most controversial idea I have for GOC but if GOC is given more ability in certain aspects it would would provide something for more combative players to do while making hostilities from GOC a larger threat and letting foundation or CI retaliate to GOC with hostilities of their own. This would allow GOC, CI or Foundation command to go to war for extended periods of time without needing prior approval from SL, this can be restricted to certain scenarios or it can be a 24/7 constant war if they wish, it is 100% down to how diplomatic talks go and the wishes of GOC command and other relevant parties. GOC would essentially be a less disruptive, more ethical CI when it comes to raiding foundation, with raid goals being stuff that doesn't interrupt the main foundation gameplay in order to prevent ruining research RP and such. Giving GOC specific raid objectives such as raiding a storage room then exfilling in order to give suit parts or ADBing a specific SCP for suit parts would be less disruptive than CI raids while providing more for foundation MTFs to do. They can also be given some more warfund stuff to make them a bit more of a threat too, as they should be. To allow for GOC R&D to continue, GOC assessment teams should be as non-combative as possible and have a reduced cooldown. Id go as far as saying they should even have easy access to a researcher outfit and card in order to perform tests, however that can possibly be done. To me this makes way more sense than the foundation just letting GOC perform tests on their SCPs, GOC should have to do it in a covert fashion, this could even contribute to the acquisition of suits.
As far as diplomacy goes, the main aspect of diplomacy should move from wars and "hey can we go into your site" to ceasefires. If GOC becomes a large enough threat ceasefires would have to be arranged to prevent things like the weaponisation of SCPs by CI or anything along those lines. This allows diplomacy to continue, albeit in a different direction, there is enough of an incentive to form ceasefires for both sides as long as they both want to prevent the same problem.
If GOC is seen as being too disruptive on surface obviously ceasefires can be arranged which all comes down to diplomacy, though GOC and foundation should have the ability to be in full control of how relations work, if that means GOC KOS foundation 100% of the time and vice versa so be it, essentially SL just dont interfere and GOC are incentivised to raid through obtaining suit materials from MRs and easier, more passive assessment teams allowing GOC to perform all their R&D RP without foundation explicitly giving them permission.
Give the GOC gamified surface activities.
Wipe the GOCs warfunds to zero and balance the amount of warfunds you get from completing the pretty basic objectives. Add more gamified objectives to surface that provide warfunds, essentially just a list of tasks members of GOC can perform that gives warfunds. Some of these tasks could also provide other things other than warfunds like other GOC suits if they manage to raid and hold some warehouses.
Give them some supply missions where they have to resupply their FOB or Pinewood, they have to destroy some documents that could break the veil, they have to do some little minigames to get some sort of tangible bonus.
Add a seperate "branch" of GOC.
To compensate for GOC now being KOS on surface by default for both CI and Foundation they require new jobs under the guise of UN peacekeepers to allow them to do their job of protecting the veil. This is as simple as creating new jobs armed with at most a pistol. While they are still technically GOC, foundation should be effectively banned from shooting them and CI should be heavily discouraged. They posess the bare minimum knowledge required to perform their duties and they are seperate lives to the normal GOC characters, meaning if the GOC colonel is kidnapped while on the UN peacekeeper job they don't know the same things the colonel does. Essentially just a ranger 2.0 with more knowledge and power but not as much as a standard GOC operative.