It looks like you already have most of the necessary things. Since you wanted feed back, here is a list I compiled for what I would want in a store:
1. Speed loaders
2. 6mm metal bushings
3. Pistons! Pistons! Pistons! Element polycarb Orange pistons, Azimuth pistons, Guarder polycarb pistons, even stock marui/jg pistons.
4. Tight bore barrels: Prometheus and Madbull 6.03's.
5. Upgrade springs
6. M4/M16 midcaps: G&P 130 round or S. Arms 160 round midcaps
7. Hopup buckings + nubs: Systema black soft bucking, Guarder clear soft, madbull blue bucking, element h-nubs, Shredders concave spacer.
8. Metal 1 piece hopup chambers for M4/m16: Prometheus neo strike chamber, Madbull 3 in 1 ultimate hopup chamber, G&P, etc
9. M4/M16 metal body kits and RIS kits
10. M14/Version 7 gearbox and parts
11. version 2 and version 3 spring guides, tappet plates, air nozzles, trigger switch/wire harness, and gearbox shells.
12. Standard ratio V2/3 gear set
13. Propane adapters
14. Mini-tamiya small connectors (the connectors on mini batteries)
15. battery chargers
16. 1 point bungee slings, 3 point rifle slings
17. Pistol holsters, drop leg and molle attachment holsters
18. OD, Woodland, ACU, and Multicam MOLLE pouches made by: Condor, SDS, Tactical Tailor
19. Grenade launchers and shells
20. AEG/Spring sniper: Classic Army X series m15a4s, KWA M4/M16's, JG/ECHO1 m4/m16's, G&G Proline, ICS M4/m16, CYMA M14, JG Bar 10
Yea it's a long list, it's all I can think of for now.