If you give that building to Gondor you should balance it to other factions ( i think Rohan dont need that, coz they have Hama passive. ) But Argaon have athelas, so... But i sure think Gondor heros are weaker than the others, so litle buff for Gondor should be nice, and Your idea fits that verry well. So buff for base health regeneration for other faction heros, and give Healing Hall to Gondor. Generaly i think base hp regen is really bad for heroes, I think if he isn't in combat for 1,5-2 min he could get his 100% hp.