The humorous use of -itis:
nouns denoting abnormal states or conditions, excesses, tendencies, obsessions, etc. (telephonitis; baseballitis).
For baseball, I suppose 野球【やきゅう】バカ might work, but スマホ中毒【ちゅうどく】 seems too strong, I've never heard random nouns being suffixed with 病【びょう】 or 症【しょう】, but is that a thing in Japanese?
We have another suffix 狂｛きょう｝ in addition to your バカ and 中毒｛ちゅうどく｝.
"野球狂{やきゅうきょう｝の詩｛うた｝" is a famous Japanese baseball manga series by Shinji Mizushima. One of the protagonists in a manga is a 50 years old baseball player who still throws in a professional league. Even though the 53 years old ballplayer had been rough, lost 53 points and threw 688 balls in 9th innings, he completed his retirement game. And he cancels the retirement, since he likes baseball.
We have another word "風狂｛ふうきょう｝" for zen-monks' an extraordinary behavior.
"厨二病{ちゅうにびょう｝" is a typical word for a self-conscious behavior which is typical for a teenager "everyone is looking at you".
"スマホ中毒{ちゅうどく}" does not sound pathological at least for me. "中毒{ちゅうどく｝" in daily usage, at least to me, does not imply a disease.
I can't find "症｛しょう｝" for "-itis" in a daily usage. Maybe there are some or only for a pathology.
I think -病 and -症 work pretty well. They have the same humorous allusion of the -itis suffix to a diagnosable pathological condition. Between -病 and -症, words suffixed with -病 may be more recognizable, especially when spoken, and therefore more common. I don't think there's any discernible difference in meaning when used in this way. As expected, Google shows people have used the terms "野球病/症" and "スマホ病/症" to name "diseases" caused by or connected with baseball and smartphones (the ones with -症 apparently a lot less common, also expected).
-バカ and -中毒 are conventional -- maybe too conventional; they relatively lack color and humorous effect. (Not to mention that -バカ shares some of the pejorative connotation with its lexical counsin バカ.) Also their meanings are probably more limited . They denote excessive devotion and obsession/addiction while -病 and -症 suggests obsession/addiction plus a wider range of potential abnormalities specific to the thing named by the root word. For example, from what I can tell from Google search, 野球病 mostly means obsession with the game (and potentially various symptoms stemming from it) and sites that warns against the スマホ病 included physical conditions like the "text thumb", tiredness of the eye, nausea and lack of sleep.
Which reminds me of another relevant word that can be used in combinations -- "症候群". But this is more like (or the translation for? ) "syndrome". (Japanese Wikipedia points out that there really isn't a clear distinction between "症候群" and "-病" though.)
Wataru 'Watson' Subridgeさんのコメントにあった「～狂｛ぐる｝い」が -itis に完全に相当するとは言いませんが古風{こふう}な言い方で好きです。
辞書によると「～狂｛ぐる｝い」次のように定義されております。
ぐるひ 【狂い】
《名詞に付けて》 …に夢中｛むちゅう｝ではなはだしい状態｛じょうたい｝、または人｛ひと｝。 「女｛おんな｝狂｛ぐる｝い」
辞書の例にある「女狂い infatuation (with a woman); to be completely obsessed with a woman; to have an insane passion for a girl"」が使用例として有名ですね。使用例も一番多いと思います。
若干{じゃっかん}古風な表現ですので、「スマホ」、「パソコン」、「ゲーム」等では「～狂｛ぐる｝い」をつけてもしっくりきませんが、「競馬｛けいば｝ 狂い a craze for horse racing」のように、「パチンコ」「競艇｛きょうてい｝」「競輪｛けいりん｝」「麻雀/マージャン」とは相性｛あいしょう｝が良いと思います。基本的にギャンブルの世界ですね。ふしだらな男性の道楽｛どうらく｝である「飲む、打つ、買う」の内、「女狂い」は「買う」、ギャンブルは「打つ」ですが、酒を「飲む」ことでは「～狂い」はそれほど聞きません。
次のところに「～狂｛ぐる｝い」の例がたくさんありますが、女につながる「色事 a love affair or sensual pleasures」の例が多いですね。しかし、私が無いと思っていた「酒狂い」もちゃんとありますのは驚きです。普通「酒狂い」は何と言うのでしょうか。「大酒｛おおざけ｝飲み」あるいは「呑兵衛｛のんべえ｝」と言うのが普通でしょうか。
全部とは言いませんが、「～狂いの人」に言えるのは、そのような人は周りの人、特に家族に大変迷惑をかけていると思います。
"妾｛めかけ｝ 狂い concubine or mistress", "芸者｛げいしゃ｝狂い", "吉原｛よしわら｝狂い", "酒狂い", "電脳｛でんのう｝ 狂い computer", "釣魚｛ちょうぎょ｝ 狂い fishing", "遊女{ゆうじょ}狂い", "浮気{うわき} 狂い affair or a bit on the side cheating", "役者{やくしゃ}狂い a woman runs mad after an actor", "女狂い", "賭博{とばく} 狂い gambling", "間夫{まぶ}狂い a prostitute being crazy about her lover", "色{いろ} 狂い sensuality", "名聞{みょうもん} 狂い reputation", "比丘尼{びくに} 狂い a prostitute wearing a nun's costume" and "万年青｛おもと｝ 狂い Japanese rhodea (plant of the lily family)"
-itis にどれほど似ているか自信はありませんが、英語の fetish から派生したと思われる「～フェチ」も、質問者の期待している日本語の接尾語の一つではないでしょうか。興味があれば是非調べてください。
