First of all, this is more of a theory-crafting idea. Obviously we won’t know 100% what to do until Rank10 are released. The purpose of this thread is for me to see what other classes synergize well with certain builds, but just not enough space to be fitted in a build. For builds that have rank8 classes as core, tell me if you would go C2-C3 or not. I will start with few of my builds as example. Feel free to provide your builds and ideas as well, thanks.
Full INT, cleric2-priest3-chap-PD3. Rank10 will be kabbalist. Really need the SP recovery from Ein Sof.
Full SPR, wiz-pyro-link-thaum3-sorc-shadownancer2. Rank10 will be shadownancer3. Even though we don’t know what SM3 provides, higher level of the 3 offensive spells and hullucination are important.
STR/DEX, Arch2-quarrel3-falc3-mergen. Rank10 I don’t know… Rogue?
Full STR, Arch2-sap2-wugu2-cannoneer3. Rank10 I don’t know… Sap3 or rogue?
Full STR, sword3-highland2-doppel3-shinobi. Rank10=?
Full SPR, wiz-pyro-linker2-chrono3-sage2. Rank10=? Judging from current, definitely not sage3.
Full STR, sword-pelt-hop3-doppel-dragoon3. Rank10=?
Full INT, wiz-pyro2-cryo2-psycho3-rune caster. Rank10=?
Full STR, cleric2-priest2-monk2-kabba-zealot2. Rank10 will be zealot3. Again, we don’t know what C3 will be like, but the C1-2 skills are insanely strong and worth higher level.

Pyro2 is a choice too. Fire Pillar goes well with Dimension Compression to disable multiple enemies. Also, if you consider the Wiz rebalance, Wiz2 will be important since Sleep attribute to apply bonus damage with Psychokinesis/Non-Elemental attacks is a C2 attribute iirc.