It's been over 3 years :)

The common module was designed to prevent the internal modules depending on the core plugin. With the introduction of localization, this overcomplication became ever more exacerbated.
Probably will play around a bit more to remove freshly introduced static abuse before release.
Closes #61

plugin/src/main/java/com/lishid/openinv/internal/IPlayerDataManager.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright (C) 2011-2020 lishid. All rights reserved.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 3.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.lishid.openinv.internal;
+
+import org.bukkit.OfflinePlayer;
+import org.bukkit.entity.Player;
+import org.bukkit.inventory.InventoryView;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+public interface IPlayerDataManager {
+
+    /**
+     * Loads a Player for an OfflinePlayer.
+     * </p>
+     * This method is potentially blocking, and should not be called on the main thread.
+     *
+     * @param offline the OfflinePlayer
+     * @return the Player loaded
+     */
+    @Nullable
+    Player loadPlayer(@NotNull OfflinePlayer offline);
+
+    /**
+     * Opens an ISpecialInventory for a Player.
+     *
+     * @param player the Player opening the ISpecialInventory
+     * @param inventory the Inventory
+     *`
+     * @return the InventoryView opened
+     */
+    @Nullable
+    InventoryView openInventory(@NotNull Player player, @NotNull ISpecialInventory inventory);
+
+    void sendSystemMessage(@NotNull Player player, @NotNull String message);
+
+    @NotNull
+    default String getLocale(Player player) {
+        return player.getLocale();
+    }
+
+}