G: TL;DR
Go sharp + acidic, not creamy. Mustard, vinegar, or peppery heat will balance the fat and the sweet roll.

Good pairings (no mayo):

• Dijon mustard or whole-grain mustard – classic with both salami and Swiss; cuts the fat cleanly.
• Spicy brown mustard – same logic, a little more bite.
• Red wine vinegar (a light splash) or oil + vinegar – especially good if you add tomatoes; keeps it bright.
• Horseradish (straight or mustard-mixed) – sharp, sinus-clear bite that works extremely well with Swiss.
• Pickled elements if you have them: banana peppers, pepperoncini, pickled onions – acidity + crunch.
• Hot sauce (vinegar-forward, not sweet) – think Tabasco-style rather than sriracha.
• Black pepper + olive oil – simple, old-school deli move that actually works here.

If you want one “assembled” suggestion:
Tomato + Swiss + salami + Dijon + a few drops of red wine vinegar + black pepper.
It stays crisp, salty, and sharp without going creamy.

Japanese note (simple, just once):
酸っぱい 味（あじ） が 合（あ）う。 — Sour flavors fit well here.

This sandwich wants contrast, not cushioning.

G: Yes.
Tomato absolutely works.

It does three useful things at once:

• Acidity → cuts the fat of salami and Swiss.
• Juiciness → offsets the dry, cured meat without needing mayo.
• Freshness → keeps the sweet roll from tipping the whole thing into “heavy.”

If you want to lock it in without adding creaminess:

• Tomato + salt (important)
• Tomato + black pepper
• Tomato + mustard or a few drops of vinegar

Japanese (simple, one line):
トマト は 合（あ）う。 — Tomato works.

That combo is structurally sound.